Section 18.1 18.1 Electromagnetic...

18.1 Electromagnetic Waves

Reading StrategyComparing and Contrasting Copy thetable below. As you read about electro-magnetic waves, fill in the table to comparethem with mechanical waves. Use E forproperties of electromagnetic waves, M formechanical waves, and B for both.

Key ConceptsHow are electromagneticwaves different frommechanical waves?

What is the maximumspeed of light?

How do electromagneticwaves differ from oneanother?

What is the dual nature ofelectromagnetic radiation?

What happens as lighttravels farther from itssource?

Vocabulary◆ electromagnetic

waves◆ electric field◆ magnetic field◆ electromagnetic

radiation◆ photoelectric effect◆ photons◆ intensity

What do X-ray machines, microwave ovens, and heat lamps havein common with police radar, television, and radiation therapy? Theyall use waves. You are surrounded by such waves all the time. But youmay not realize it, because most waves are invisible.

With X-rays, you can take pictures of your bones. Your dentist usesX-rays to examine the inner structure of your teeth. Microwaves cook orreheat your meals and carry cell phone conversations between you and

your friends. Radio waves bring your favorite music to your radiofrom the radio station. Ultraviolet rays can give you a sunburn.

Without waves, the girl in Figure 1 wouldn’t be able totalk with her friends on a cell phone. Without waves,

you wouldn’t be able to watch your favorite TVshow. You wouldn’t be able to see colors. In fact,

without waves you wouldn’t be able to see any-thing at all.

Travels through vacuum

Travels through medium

Fits wave model

Fits particle model

Transverse wave

Longitudinal wave

a. ?

E

B

b. ?

c. ?

d. ?

Figure 1 The waves that carry this girl’scell phone conversation are not visible.The girl may not even know they exist.But their existence is what makes cellphone technology possible.

532 Chapter 18

532 Chapter 18

FOCUS

Objectives18.1.1 Describe the characteristics of

electromagnetic waves in avacuum and how Michelsonmeasured the speed of light.

18.1.2 Calculate the wavelength andfrequency of an electromag-netic wave given its speed.

18.1.3 Describe the evidence for thedual nature of electromagneticradiation.

18.1.4 Describe how the intensity oflight changes with distancefrom a light source.

Build VocabularyWord-Part Analysis Ask studentswhat words they know that have the keyword parts electro, magnet, and photo.(electricity, magnet, and photograph)Give a definition of each word part.(Electro means “shining,” magnet means“attracting like material,” and photomeans “light.”) Give additional examplesthat share the word parts in question(electron, magnetism, photocopy).

Reading Strategya. B b. E c. B d. M

L2

L2

Reading Focus

1

Section 18.1

Print• Reading and Study Workbook With

Math Support, Section 18.1 and Math Skill: Calculating Wavelength andFrequency

• Math Skills and Problem SolvingWorkbook, Section 18.1

• Transparencies, Chapter Pretest andSection 18.1

Technology• Interactive Textbook, Section 18.1• Presentation Pro CD-ROM, Chapter Pretest

and Section 18.1• Go Online, NSTA SciLinks, Waves

Section Resources

Directionof

wave

Electricfield

Magneticfield

Figure 2 Electromagnetic wavesconsist of changing electric fieldsand magnetic fields. The fieldsare at right angles to each otherand to the direction of the wave.Interpreting Diagrams Howcan you tell that electromagneticwaves are transverse waves?

The Electromagnetic Spectrum and Light 533

What Are Electromagnetic Waves?The visible and invisible waves you will learn about in this chapter exhibitsome of the same behaviors as mechanical waves. Other behaviors areunique to electromagnetic waves. Electromagnetic waves are transversewaves consisting of changing electric fields and changing magnetic fields.Like mechanical waves, electromagnetic waves carry energy from placeto place. Electromagnetic waves differ from mechanical waves in howthey are produced and how they travel.

How They Are Produced Electromagnetic waves are pro-duced by constantly changing fields. An electric field in a region ofspace exerts electric forces on charged particles. Electric fields are pro-duced by electrically charged particles and by changing magnetic fields.A magnetic field in a region of space produces magnetic forces.Magnetic fields are produced by magnets, by changing electric fields,and by vibrating charges. Electromagnetic waves are producedwhen an electric charge vibrates or accelerates. Figure 2 shows thatthe fields are at right angles to each other.You can tell this is a transversewave because the fields are also at right angles to the direction in whichthe wave travels.

How They Travel Because changing electric fields produce chang-ing magnetic fields, and changing magnetic fields produce changingelectric fields, the fields regenerate each other. As the fields regenerate,their energy travels in the form of a wave. Unlike mechanical waves,electromagnetic waves do not need a medium. Electromagneticwaves can travel through a vacuum, or empty space, as well as throughmatter. The transfer of energy by electromagnetic waves travelingthrough matter or across space is called electromagnetic radiation.

What are electromagnetic waves?

For: Links on waves

Visit: www.SciLinks.org

Web Code: ccn-2181

INSTRUCT

What AreElectromagneticWaves?Use VisualsFigure 2 Emphasize that an electro-magnetic (EM) wave consists of both anelectric field and a magnetic field. Havestudents note that the electric field andmagnetic field vibrate in planes that areperpendicular to each other (shown bythe yellow areas). Ask, In which directionis the EM wave in the figure traveling?(To the right) In which direction are thedisturbances in the electric andmagnetic fields traveling? (Perpendicularto the direction of the EM wave) Why can’tthe electric field wave exist without themagnetic field wave? (Each field producesthe other—if one is present so must be theother.) How is the EM wave differentfrom a wave traveling along a rope?How are they the same? (The EM wavedoes not need a medium to travel through,whereas the wave in the rope could not existif the rope were not present. Also, the EMwave is made up of electric and magneticwaves, whereas the wave in the rope has a single component. Both waves aretransverse waves.)Visual

L1

2


Customize for English Language Learners

Simplify the Presentation The large number of vocabulary words in thissection is a challenge to an English languagelearner. Help ease the challenge by tailoringyour teaching presentation of the sectioncontent to the less-proficient English skills ofyour students. Do this by speaking directly and

simplifying the words and sentence structuresused to explain the material. Use bodylanguage when appropriate to emphasizeimportant words. Restate complicatedsentences into shorter phrases that make useof more common words.

Answer to . . .

Figure 2 They are transverse becausetheir fields are at right angles to thedirection in which they are traveling.

Electromagnetic wavesare transverse waves

consisting of changing electric fieldsand changing magnetic fields.

Download a worksheet on wavesfor students to complete, and findadditional teacher support fromNSTA SciLinks.

0530_hsps09te_Ch18.qxp 4/19/07 9:21 AM Page 533

Mt. San Antonio

Light source

Octagonalrotating mirror

Telescope

Mirror

Mt. Wilson

35.4 km

534 Chapter 18

The Speed of Electromagnetic WavesA thunderstorm is approaching. The sky is dark, and lightning flashesin the distance. Within a few seconds, you hear thunder’s low rumble.As the storm approaches, the lightning gets brighter and the thunderlouder. The lightning flashes and the sound of thunder come closer intime. Still, you see the lightning before you hear the thunder, becauselight travels faster than sound. But how much faster is light?

Michelson’s Experiment In ancient times, people tried to meas-ure the speed of light but no instrument was accurate enough. Lightmoves so fast that people thought its speed was infinite. Several experi-ments in the 1800s proved it was not infinite and gave approximate values.Then, in 1926, the American physicist Albert Michelson (1852–1931)measured the speed of light more accurately than ever before.

Figure 3 shows an experimental setup similar to Michelson’s. Ontop of Mount Wilson in California, Michelson placed an eight-sidedrotating mirror. He placed another mirror, this one stationary, onMount San Antonio, 35.4 kilometers away. Michelson shined a brightlight at one face of the rotating mirror. The light reflected to the sta-tionary mirror on the other mountain and then back to Mount Wilson,where it struck another face of the rotating mirror. Michelson knewhow fast the eight-sided mirror was rotating and how far the lighttraveled from mountain to mountain and back again. With thosevalues he was able to calculate the speed of light quite accurately. Hisfindings were similar to modern measurements.

The Speed of Light Since Michelson, many other scientists havemeasured the speed of light. Their experiments have confirmed thatlight and all electromagnetic waves travel at the same speed when in avacuum, regardless of the observer’s motion. The speed of light in a vacuum, c, is 3.00 � 108 meters per second.

Figure 3 Michelson timed a lightbeam as it traveled from onemountain to another and backagain. His experiment measuredthe speed of light more accuratelythan it had been measured before.Inferring Why must the lightbeam travel so far for its speed tobe measurable?

534 Chapter 18

The Speed ofElectromagneticWavesBuild Reading LiteracySequence Refer to page 290D inChapter 10, which provides the guidelines for a sequence.

Have students read the text on page 534 related to Michelson’sexperiment. Then, have students do the following:1. Create a sketch of Michelson’s experi-mental setup as shown in Figure 3. Thesketch should include all labels shown in Figure 3.2. Number and describe the steps thelight follows on its path through theexperimental setup. Students shouldstart with the light emitted by the lightsource as Step 1.3. Student sketches should include as much detail as they can find in thetext, Figure 3, and its caption. Visual, Portfolio

FYIMichelson’s work earned him a NobelPrize in Physics, the first ever awarded toan American.

L1

Section 18.1 (continued)

Speed of Light Due to the definition of themeter, the speed of light has an exact value.The meter is defined as the distance light travels in a vacuum in of a second.Thus, the speed of light has an exact value of299,792,458 m/s.

To imagine the speed of light, considerdriving non-stop at 60 miles per hour fromNew York City to San Francisco. This trip wouldtake you about 50 hours (a little more than two days). Light travels this distance in less thantwo-hundredths of a second (0.02 second).

1299,752,458

Facts and Figures

Wavelength and FrequencyIn a vacuum, all electromagnetic waves travel at the same speed. Butnot all electromagnetic waves are the same. Electromagneticwaves vary in wavelength and frequency.

The speed of an electromagnetic wave is the product of its wave-length and its frequency. Because the speed of electromagnetic wavesin a vacuum is constant, the wavelength is inversely proportional tothe frequency. As the wavelength increases, the frequency decreases. Ifyou know the wavelength of an electromagnetic wave, you can calcu-late its frequency.

Calculating Wave SpeedA radio station broadcasts a radio wave with a wavelength of3.0 meters. What is the frequency of the wave?

Read and UnderstandWhat information are you given?

Speed � c � 3.00 � 108 m/s

Wavelength � 3.0 m

Plan and SolveWhat unknown are you trying to calculate?

Frequency � ?

What formula contains the given quantities andthe unknown?

Speed � Wavelength � Frequency

or, Frequency �

Replace each variable with its known value.

Frequency �

� 1.0 � 108 Hz

Look Back and CheckIs your answer reasonable?

Check that product of wavelength and frequency gives aspeed of 3.0 � 108 m/s.

Speed � 3.0 m � (1.0 � 108 Hz) � 3.0 � 108 m�s

SpeedWavelength

1. A global positioning satellitetransmits a radio wave witha wavelength of 19 cm. What isthe frequency of the radio wave?(Hint: Convert the wavelengthto meters before calculatingthe frequency.)

2. The radio waves of a particularAM radio station vibrate680,000 times per second. Whatis the wavelength of the wave?

3. Radio waves that vibrate160,000,000 times per secondare used on some train linesfor communications. If radiowaves that vibrate half as manytimes per second were usedinstead, how would thewavelength change?


3.00 � 108 m/s3.0 m


Answer to . . .

Figure 3 The time interval would betoo small to measure if the distancewere shorter.

Wavelength andFrequencyBuild Math SkillsFormulas and Equations Havestudents read the text following theheading Wavelength and Frequency. Ask them to use the description given towrite the wavelength-frequency formula(Speed � Frequency � Wavelength).Once they have written the correctformula, have them practice solving it foreach of the three possible unknowns—speed, wavelength, and frequency.Logical, Portfolio

Direct students to the Math Skills in the Skills and Reference Handbookat the end of the student text for addi-tional help.

Solutions1. Speed � Wavelength � Frequency;Frequency � Speed/Wavelength �(3.00 � 108 m/s)/(0.19 m) �1.6 � 109 Hz2. Speed � Wavelength � Frequency;Wavelength � Speed/Frequency �(3.00 � 108 m/s)/(680,000 Hz) � 440 m3. At 160 MHz: Wavelength �Speed/Frequency � (3.00 � 108 m/s)/(160,000,000 Hz) � 1.9 m; at 80 MHz:Wavelength � Speed/Frequency �(3.00 � 108 m/s)/(80,000,000 Hz) �3.8 m; The wavelength would be 1.9 meters longer. Logical

For Extra HelpMake sure students’ first step is to writethe formula required to solve eachproblem. Then, check that they are ableto solve the formula for the unknownvariable using basic algebra skills. Logical

Additional Problems1. What is the frequency of an electro-magnetic wave that has a wavelength of 2.0 m? (1.5 � 108 Hz)2. What is the wavelength of an electro-magnetic wave that has a frequency of1.5 � 1010 Hz? (0.020 m)Logical, Portfolio

L1

L2

L1


Interferencepattern appearson screen.

Dark bands showdestructive interference.

Bright bands showconstructive interference.

Light from singleslit produces coherentlight at second card.

Lightsource

Card withone slit

Card withtwo slits

536 Chapter 18

Figure 5 This diagram illustratesYoung’s experiment, whichshowed that light behaves like awave. When light passes througha single slit and then a double slit,it produces an interferencepattern. Constructive interferenceproduces bright bands of light.Destructive interference producesdark bands.Predicting What would you expect to see on the screen if light behaved like a stream of particles?

Wave or Particle?Scientists know that electromagnetic radiation travels as a wave.Scientists also have evidence that electromagnetic radiation behaveslike a stream of particles. In the late 1600s, the English physicist IsaacNewton was the first to propose a particle explanation. He based thishypothesis on two pieces of evidence: light travels in a straight line andit casts a shadow, as shown in Figure 4. But not all evidence supportsNewton’s hypothesis. So which is light, wave or particle? It is both.

Electromagnetic radiation behaves sometimes like a wave andsometimes like a stream of particles.

Evidence for the Wave Model In 1801, the English physi-cist Thomas Young (1773–1829) showed that light behaves like a wave.Look at Figure 5. Young passed a beam of light first through a single slitand then through a double slit. Where light from the two slits reacheda darkened screen, Young observed alternating bright and dark bands.The bands were evidence that the light had produced an interferencepattern. Bright bands indicated constructive interference, and darkbands indicated destructive interference. Interference occurs only whentwo or more waves overlap. Therefore, Young’s experiment showed thatlight behaves like a wave.

What is the evidence that light travels like a wave?

Figure 4 The fact that lightcasts a shadow has been usedas evidence for both the wavemodel of light and the particlemodel of light.

536 Chapter 18

Wave or Particle?Use VisualsFigure 5 Have students observe thepattern the light makes on each of thethree cards shown. Ask, Why are there no interference patterns on the cardwith two slits? (For interference patterns to occur, there must be at least two over-lapping wave sources. The vertical slit in thefirst card provides only a single light source toilluminate the second card.) Why does thedouble-slit card produce an interferencepattern? (Each slit acts as a source of lightwaves. As the light waves spread from eachsource, they overlap and create interference.)What occurs between the areas ofmaximum brightness and maximumdarkness? (Partial interference occurs,resulting in areas of brightness between thetwo extremes.) Visual, Logical

FYIA diffraction grating uses diffraction andinterference to create a rainbow. At eachangle of transmission, only certainfrequencies interfere constructively.

L1



Evidence for the Particle Model When dim blue light hitsthe surface of a metal such as cesium, an electron is emitted. A brighterblue light causes even more electrons to be emitted, as you can see inFigure 6. But red light, no matter how bright it is, does not cause theemission of any electrons in this particular metal.

The emission of electrons from a metal caused by light strikingthe metal is called the photoelectric effect. Discovered in 1887, thephotoelectric effect was puzzling. Scientists did not understand whydim blue light caused electrons to be emitted from metal but evenbright red light did not.

In 1905, Albert Einstein (1879–1955) proposed that light, and allelectromagnetic radiation, consists of packets of energy. These packetsof electromagnetic energy are now called photons (FOH tawnz). Eachphoton’s energy is proportional to the frequency of the light. Thegreater the frequency of an electromagnetic wave, the more energyeach of its photons has.

Blue light has a higher frequency than red light, so photons of bluelight have more energy than photons of red light. Blue light consists ofphotons that have enough energy to cause electrons to be emitted froma metal surface. So blue light can cause emission of electrons.

Red light has a lower frequency than blue light, so photons of redlight have less energy than photons of blue light. Red light consists ofphotons that have too little energy to cause any electrons to be emittedfrom a metal surface. So red light does not cause emission of electrons.

What is the photoelectric effect?

Brightred light

orinfrared rays

Metalplate

Metalplate

Dim blue lightor

ultravioletrays

A No electronsare emitted.

B Electronsare emitted.

Figure 6 The emissions of electrons from a metal caused by light striking themetal is called the photoelectric effect. A Red light or infrared rays, no matterhow bright, does not cause electrons to be emitted from this metal surface. B When blue light or ultraviolet rays strike the metal surface, electrons areemitted, even if the light is dim.

Build Science SkillsInferring Have students look at Figure 6A. Ask, What happens to thered light after it strikes the metalplate and fails to produce thephotoelectric effect? (Some studentsmay not realize that the red light cannotsimply disappear. Lead students to realizethat the red light is partially reflected andpartially absorbed. The absorbed lightresults in an increase in temperature of the metal.) Logical, Visual

The Photoelectric Effect

Purpose Students indirectly observethe photoelectric effect.

Materials electroscope, small strip ofzinc, rubber rod, wool, incandescentand UV light sources

Procedure Tell students that the leavesof the electroscope can be used to showwhen a charge is present. Attach thezinc strip to the electroscope. Rub therubber rod with the wool and touch it tothe zinc strip. The zinc is now chargedand the leaves of the electroscopeshould be separated. Now shineincandescent light on the zinc strip.Repeat using the UV light source. Havestudents observe the results.

Expected Outcome When theincandescent light shines on the zinc,nothing happens. The reason for this isbecause the photoelectric effect does notoccur in the zinc when it is illuminated bylow-energy visible light photons. Whenthe UV light shines on the zinc, the leavesof the electroscope collapse and touchone another. This occurs because the UVlight source has enough high-energyphotons to induce the photoelectriceffect in the zinc. As electrons are ejectedfrom the zinc, the positive charge on theleaves is neutralized. Visual, Logical

L2

L2


FYIPhotons, which are electrically neutral, have a“rest mass” of zero. Technically, however, allphotons have energy, and thus all photonshave mass. The energy of an individual photonis given by the equation

E � hf

In this equation, h is Planck’s constant (6.6 � 10�34 J•s), and f is the frequency of the photon. The equation shows that as thefrequency of the photon increases, so does itsenergy. This is why higher-frequency, higher-energy blue light is able to produce the photo-electric effect shown in Figure 6B, whereas thelower-frequency, lower-energy red light is not.

Answer to . . .

Figure 5 You would expect to see two bright lines on the screen.

Interference only occurswith waves.

The photoelectric effect isthe emission of electrons

from a metal, caused by light strikingthe metal.


538 Chapter 18

Section 18.1 Assessment

Reviewing Concepts1. What produces electromagnetic waves?

2. How fast does light travel in a vacuum?

3. What makes electromagnetic wavesdifferent from one another?

4. Explain how light behaves like a streamof particles.

5. What happens to the intensity of light asphotons move away from the light source?

6. How does photon energy relate to frequency?

Critical Thinking7. Applying Concepts Why does blue light

cause emission of electrons from metal whilered light does not?

8. Observing Describe what happens as youget closer to a light source. Explain thisobservation.

IntensityThe closer you are to a source of light, the brighter the lightappears. If you want to read at night, you must sit near a lamp.At night, as you walk away from a street light, the area aroundyou becomes darker. A street light doesn’t give off less lightwhen you move farther from it. It just provides you with lesslight the farther away you are. Photons travel outward from alight source in all directions. Near the light source, the pho-tons spread through a small area, so the light is intense.Intensity is the rate at which a wave’s energy flows through agiven unit of area. You can think of intensity as brightness.Farther from the source, the photons spread over a larger area.

The intensity of light decreases as photons travel fartherfrom the source.

A can of spray paint can help you model a change in lightintensity. Look at Figure 7. When the nozzle is close to a piece ofpaper, the paint forms a small, dense spot. When the nozzle is far-ther from the paper, the paint forms a larger, fainter spot becausethe paint is sprayed over a larger area. Like paint on paper, lightintensity decreases as distance from the light source increases.

A wave model for light also explains how intensity decreaseswith distance from a source. As waves travel away from the source,they pass through a larger and larger area. Because the total energydoes not change, the wave’s intensity decreases.

9. What is the wavelength of an AMradio wave in a vacuum if itsfrequency is 810 kilohertz?

10. A global positioning satellite (GPS)transmits a signal at a frequency of1575 megahertz. What is thewavelength? (Hint: Assume the wavespeed is the same as in a vacuum.)

B

A

Figure 7 The closer you areto a surface when you spraypaint it, the smaller the areathe paint covers and the moreintense the paint color looks.Using Models How does acan of spray paint help youmodel a change in lightintensity?

IntensityBuild Science Skills

Observing

Purpose Students observe light intensity.

Materials low-wattage incandescentbulb, light source

Class Time 10 minutes

Procedure Darken the room and turnon the bulb. Ask for several volunteers towalk around the room and observe ifthe light coming from the bulb can beseen in all parts of the room. Also, havethem note the brightness of the bulb atvarious distances from the bulb.

Expected Outcome Students willobserve that the light from the bulb willradiate out in all directions. They also willobserve that the brightness of the lightdecreases as students move farther fromthe bulb. Visual, Kinesthetic, Group

ASSESSEvaluate UnderstandingHave students write three math problems(with solutions) based on the frequency-wavelength formula. Have them analyzeand solve the problems in class.

ReteachUse Figure 2 to summarize the key features of an electromagnetic wave.

Solutions9. Wavelength � Speed/Frequency �(3.00 � 108 m/s)/(810,000 Hz) � 370 m10. Wavelength � Speed/Frequency �(3.00 � 108 m/s)/(1.575 � 109 Hz) �0.190 m

If your class subscribesto the Interactive Textbook, use it toreview key concepts in Section 18.1.

L1

L2

3

L2


6. The greater the frequency of an electro-magnetic wave, the more energy each of itsphotons has.7. Photons of blue light have more energythan photons of red light. 8. The closer you get to a light source, the brighter the light appears because itsphotons have not spread out as much.


1. The acceleration or vibration of an electriccharge produces EM waves.2. 3.00 � 108 m/s.3. They vary in wavelength and frequency. 4. Red light photons do not cause electrons to be ejected from a particular metal as bluelight photons can.5. As photons move away from the lightsource, the intensity decreases.

Answer to . . .

Figure 7 As the can moves fartherfrom the surface, the area the paintcovers becomes larger, but less intense.The same happens when light shineson a surface.

538 Chapter 18


18.2 The Electromagnetic Spectrum

Reading StrategySummarizing Copy the chart below andadd four more rows to complete the table forthe electromagnetic spectrum. After you read,list at least two uses for each kind of wave.

Key ConceptsWhat waves are includedin the electromagneticspectrum?

How is each type ofelectromagneticwave used?

Vocabulary◆ electromagnetic

spectrum◆ amplitude

modulation◆ frequency

modulation◆ thermograms

How do you investigate something that is invisible? Firstyou have to suspect that it exists. Then you have to figure outa way to detect what is invisible and collect data about it. Suchwas the way the German-born astronomer William Herschel(1738–1822) discovered infrared radiation.

The Waves of the SpectrumIn England in 1800, with a technique discovered earlier,Herschel used a prism to separate the wavelengths present insunlight. He produced a band of colors: red, orange, yellow,green, blue, and violet. He wondered if the temperature ofeach color of light was different from the temperature of theother colors of light. As you can see in Figure 8, Herschelplaced thermometers at various places along the color bandand measured the temperatures. Herschel observed that thetemperature was lower at the blue end and higher toward thered end.

This discovery made Herschel pose a new question: Would thetemperature increase even more beyond the red end, in an area thatshowed no color? He measured the temperature just beyond the redend of the color band. This area recorded an even higher temperaturethan the red area. Herschel concluded there must be invisible radia-tion beyond the red end of the color band.

Radio Waves

Infrared Rays b. ?

Communications

Keepingfood warm

Typeof Waves

Uses

a. ?

Figure 8 Herschel measured thetemperature of different colors oflight. The temperature was lowestat the blue end and highest at thered end. Curiosity led Herschel todiscover evidence of radiationpast the red end of the band ofvisible light.

FOCUS

Objectives18.2.1 Rank and classify electromag-

netic waves based on theirfrequencies and wavelengths.

18.2.2 Describe the uses for differentwaves of the electromagneticspectrum.

Build VocabularyLINCS Have students: List the parts that they know (for example, define thermogram). Imagine a picture (create a mental picture of a thermogram). Note a sound-alike word (thermometer).Connect the terms (make up a short storyabout thermograms that uses the sound-alike word, thermometer). Self-test (quizthemselves).

Reading Strategya. Cooking and radar detection systemsb. Detecting heat differences Additional rows Visible Light: aids invision and communication; UltravioletRays: health (kill microorganisms inheating and cooling systems), agriculture(energy source to promote plant growth);X-rays: medicine, transportation(inspection tool); Gamma Rays: medicine(kill cancer cells, form images of thebrain), industry (inspection tool)

INSTRUCT

The Waves of the SpectrumBuild Reading LiteracyOutline Refer to page 156D in Chapter 6, which provides the guidelines for an outline.

Have students outline pp. 539–545.Outlines should follow the headingstructure used in the section. Majorheadings are in green, and subheadingsare in blue. Ask, Based on your outline,what are television waves, microwaves,and radar waves classified as? (Types ofradio waves) Verbal, Logical

L1

2

L2

Reading Focus

1



Math Support, Section 18.2 • Math Skills and Problem Solving

Workbook, Section 18.2• Transparencies, Section 18.2

Technology• Interactive Textbook, Section 18.2• Presentation Pro CD-ROM, Section 18.2• Go Online, NSTA SciLinks, Electromagnetic

spectrum

Section Resources

Section 18.2

PPLS

0530_hsps09te_Ch18.qxp 3/6/07 2:08 PM Page 539

Radio and

television waves

Microwave andradar waves

Infraredrays

Visiblelight

Ultra-violetrays Gammarays

SHORTHIGH

Wavelength

Frequency

Radio waves

LONG

LOW

X-rays

Today, radiation beyond the red end of the color band is calledinfrared radiation. Herschel experimented with infrared radiation andfound it had many of the same properties as visible light. With theseexperiments, Herschel opened the door to the study of invisible typesof electromagnetic radiation.

The full range of frequencies of electromagnetic radiation is calledthe electromagnetic spectrum. Figure 9 shows the spectrum of electro-magnetic radiation in order of increasing frequency from left to right.Visible light is the only part of the electromagnetic spectrum that youcan see, but it is just a small part. The electromagnetic spectrumincludes radio waves, infrared rays, visible light, ultraviolet rays,X-rays, and gamma rays. Each kind of wave is characterized by arange of wavelengths and frequencies. All of these waves have manyuseful applications.

Radio Waves Radio waves have the longest wavelengths in the electromagnetic spec-trum, from 1 millimeter to as much as thousands of kilometers orlonger. Because they are the longest waves, radio waves also have thelowest frequencies in the spectrum—300,000 megahertz (MHz) or less.

Radio waves are used in radio and television technologies, aswell as in microwave ovens and radar.

What is the full range of frequencies of electromagnetic radiation called?

Figure 9 The electromagneticspectrum consists of radio waves,infrared rays, visible light,ultraviolet rays, X-rays, andgamma rays. Interpreting Diagrams Whichwaves of the electromagneticspectrum have the longestwavelengths?

540 Chapter 18

For: Links on theelectromagneticspectrum


Web Code: ccn-2182

540 Chapter 18

FYIIn the Herschel experiment, in theory,waves at the blue end of the spectrumshould have more energy than waves at the red end because blue light has a higher frequency than red light. Thereason the red is warmer is that a prismdoes not separate the colors equally. Thered light is concentrated into a smallerarea than the blue light.

Radio WavesUse VisualsFigure 9 Emphasize that although the EM waves shown all have differentfrequencies and wavelengths, they alltravel at 3.00 � 108 m/s when in avacuum. Ask, In the electromagneticspectrum, as wavelength decreases,what happens to frequency? (Frequencyincreases.) What is the product of anyEM wave’s frequency and its wave-length in a vacuum? (The speed of light,c, or 3.00 � 108 m/s.) Which color in the visible spectrum has the highestfrequency? (Violet light) The longestwavelength? (Red light) Visual, Logical

Build Science SkillsObserving Assign groups of studentsto carry small AM/FM radios around tovarious parts of their community andlisten to the reception quality of bothAM and FM signals. Select specific AMand FM radio stations for students tolisten to. Ask students to rate the qualityof the reception on a scale from 1 to 10,where 1 is very poor reception and 10 isexcellent reception. Also have studentsdescribe the physical surroundingswhere each observation is made. Ask,Which type of radio signal is moreaffected by the location of the radio?(FM) What can you infer from yourobservations about the relativeabilities of AM and FM waves to bendaround obstacles? (AM radio waves arebetter at moving around obstacles.)Visual, Kinesthetic, Group

L3

L1


Customize for Inclusion Students

Visually Impaired Remind students of the basic wave character-istics and properties of crests, troughs,wavelength, and wave speed. Then, havestudents imagine a wave that has a wavelengthas long as a small house (~10 m). Tell them thislength describes a radio wave. Next, have them

imagine a wave that has a wavelength equal to about the width of a hand (~12 cm). Thisdescribes a microwave. Then, tell students that these electromagnetic waves travel so fastthat in one second they can go around Earthseveral times.Download a worksheet on the

electromagnetic spectrum forstudents to complete, and findadditional teacher support fromNSTA SciLinks.


Radio In a radio studio such as the one in Figure 10,music and voices that have been changed into elec-tronic signals are coded onto radio waves and thenbroadcast. There are two ways that radio stations codeand transmit information on radio waves. Both waysare based on a wave of constant frequency and ampli-tude. To code the information onto this wave so that itcan be broadcast, one of two characteristics of the wavemust be varied, or modulated.

In amplitude modulation, the amplitude of thewave is varied. The frequency remains the same. AMradio stations broadcast by amplitude modulation. Infrequency modulation, the frequency of the wave isvaried. The amplitude remains the same. FM stationsbroadcast by frequency modulation. Whichever way theradio wave is transmitted, your radio receives it, decodesit, and changes it back into sound waves you can hear.

Have you ever traveled a long distance in a car and“lost” a station on the radio? A station is lost when itssignal becomes too weak to detect. An FM radio stationis more likely to be lost than an AM station because FMradio signals do not travel as far as AM signals alongEarth’s curved surface. AM radio stations use frequen-cies between 535 kilohertz and 1605 kilohertz. FMstations use frequencies between 88 megahertz and 108megahertz. Particles in Earth’s upper atmosphere reflectthe lower-frequency AM radio waves much better thanthe higher-frequency FM radio waves. The reflectionhelps transmit AM signals farther.

Television Radio waves also carry signals for televi-sion programming. The process is like transmittingradio signals. But one difference is that the radio wavescarry information for pictures as well as for sound.Once they have been broadcast, the signals arereceived by an antenna, and sent to the TV set.

Location and weather can affect the receptionof television signals by an antenna. For that reason,many people prefer to receive television signals thathave been transmitted by satellite. With this typeof transmission, TV broadcasts are sent to satellites,which then retransmit the signals back to Earth.

If you have a satellite dish, you can receive thesignals directly. If not, a cable service can receivethe signals and resend them to your home.

A

B

Radio Broadcasting

Amplitude modulation

Frequency modulation

Figure 10 Theannouncer’s voice andthe music on CD leavethe radio studio aselectronic signals.Those signals are usedto produce a wavewith either a varyingamplitude or avarying frequency. A AM waves have avarying amplitude. B FM waves have avarying frequency.

Use Community ResourcesArrange for your class to visit a localradio station. Have students observe the equipment used to transmit live and prerecorded programs. Encouragestudents to ask questions regarding thestrength and type of signal the stationbroadcasts, the range of the signal, andthe role the surrounding landscape hasin a listener’s ability to pick up the signal.Inquire if the station also broadcasts thesignal over the Internet, and if so, howthis is done.Interpersonal, Portfolio

Radio Reception

Purpose Students observe factors thataffect radio signal reception.

Materials small portable radio with anantenna, cardboard or wooden box largeenough to contain the radio, boxlikeenclosure made of metal chicken wire

Procedure Turn on the radio, extend its antenna, and tune it to a radio station.Have students listen to the radio signal asthe radio is first inside the box, next insidethe wire enclosure with the antennainside the enclosure, and finally inside the wire enclosure, but with the antennasticking out through the enclosure. Ifpossible, repeat these steps for both AMand FM signals.

Expected Outcome The box shouldhave very little, if any, effect on recep-tion. The signal will be lost when theradio and its antenna are enclosed in thewire enclosure. The signal will returnwhen the antenna is extended outsidethe wire enclosure. Visual, Group

L2

L2


Answer to . . .

Figure 9 Radio waves have thelongest wavelengths.

The full range of frequencies of

electromagnetic radiation is called the electromagnetic spectrum.


Microwaves The shortest-wavelength radio waves are calledmicrowaves. Microwaves have wavelengths from about 1 meter toabout 1 millimeter. Their frequencies vary from about 300 megahertzto about 300,000 megahertz.

Microwaves cook and reheat food. When water or fat moleculesin the food absorb microwaves, the thermal energy of these mole-cules increases. But microwaves generally penetrate foods only a fewcentimeters, so heating occurs only near the surface of the food. Thatis why instructions tell you to let the food stand for a few minutes—so thermal energy can reach the center by conduction. Microwavesalso carry cell phone conversations. The process works much like theradio broadcast.

Radar The word radar is an acronym for radio detectionand ranging. Radar technology uses a radio transmitter tosend out short bursts of radio waves. The waves reflect offthe objects they encounter, and bounce back toward wherethey came from. The returning waves are then picked upand interpreted by a radio receiver.

Recall that the Doppler effect is an apparent changein the frequency of a wave. The Doppler effect can be usedto find the speed of a moving car. Radio waves are sentfrom a stationary source, such as the radar trailer inFigure 11, toward a moving car. The faster a car is movingtoward the source, the higher is the frequency of the radiowaves returning to the source.

542 Chapter 18

Figure 11 A speed-monitoringtrailer uses radar to measure thespeed of an approaching car. Itreminds motorists of the postedspeed limit and makes themaware of their actual speed.

How Long Does an AntennaNeed to Be?

Have you ever noticed how the lengths ofantennas vary from quite short (cell phones) tovery long (radio transmitters)? The length of anantenna depends in part on the length of thewaves it transmits. Each letter in the graph (A–E)represents an antenna of a different length. Thegraph shows the wavelengths that can betransmitted by antennas of a few different lengths.

1. Calculating What is the frequency of thewave that antenna B transmits? (Hint: Assumethe wave travels at the speed of light.)

2. Drawing Conclusions What relationship isthere between antenna length and wavelength?

3. Inferring At an outdoor concert, a singer isusing a wireless microphone with antenna C.Speakers broadcast her performance. Now andthen the speakers also broadcast an employeetaking an order at a fast food restaurantnearby. What is the approximate wavelength ofthe transmissions from the restaurant? How doyou know?

4. Predicting If you used a microphone thattransmitted waves at 600 MHz, approximatelyhow long would its antenna need to be?

Wavelength (mm)

An

ten

na

Len

gth

(mm

)

350 400 450 500 550 600

Antenna Length vs. Wavelength

0255075

100125150

A B CD

E

542 Chapter 18

How Long Does an Antenna Need to Be?Answers1. The wavelength is about 435 mm.Thus, frequency � (3.00 � 108 m/s)/(0.435 m) � 6.9 � 108 Hz � 690 MHz.2. There is an approximately linearrelationship between antenna lengthand wavelength. The wavelength isabout four or five times the antennalength. 3. The restaurant transmissions are aboutthe same wavelength as the singer’s,about 105 MHz, because the antenna ispicking up both transmissions. 4. The transmitted wavelength is about(3.00 � 108 m/s)/(6.0 � 108 Hz) �0.50 m, or 500 mm. On the graph, this is about halfway between thewavelengths used by antennas C and D. Therefore, the antenna lengthused should be about 115 mm (abouthalfway between the lengths ofantennas C and D).

For Extra HelpDiscuss the basic features of a line graph.Point out the relationship between theantenna-length variable (plotted on thevertical axis) and the wavelength variable(plotted on the horizontal axis). Reinforcethat each data point (A through E)corresponds to a unique set of horizontal-axis and vertical-axis values. Remindstudents that the wavelength shown on the graph is related to a uniquefrequency by the following equation:Speed � Wavelength � Frequency. Tell students that for this exercise, it isacceptable to use 3.00 � 108 m/s as thespeed of an electromagnetic wave. Visual

L1

L2


Microwave Cooking Most microwave ovenshave a clear door lined with a perforated metalgrid. Typically, the holes in the grid are about 2 mm in diameter. The holes are large enoughto allow you to see into the microwave oven,but small enough to prevent microwaves fromescaping. Microwaves used in cooking havewavelengths between 12 cm and 33 cm.

The reason many microwaves haveplatforms that rotate the food during cookinghas to do with interference. Microwaves insidethe oven interfere constructively anddestructively, creating hot and cold areas. Byrotating the food while it cooks, the unevencooking effect of these areas is minimized.

Facts and Figures


Infrared RaysInfrared rays have higher frequencies than radio waves and lower fre-quencies than red light. Infrared wavelengths vary from about1 millimeter to about 750 nanometers. (A nanometer is 10–9 meters,or one millionth of a millimeter.) Infrared rays are used as asource of heat and to discover areas of heat differences.

You cannot see infrared radiation, but your skin senses it as warmth.Reptile habitats at zoos are often kept warm with infrared lamps.Restaurants use infrared lamps to keep buffet-style foods at a safe tem-perature for consumption.

Warmer objects give off more infrared radiation than coolerobjects. A device called a thermograph uses infrared sensors to createthermograms. Thermograms (THUR moh gramz) are color-coded pic-tures that show variations in temperature. They are used to find placeswhere a building loses heat to the environment. Thermograms can alsolocate problems in the path of electric current, as shown in Figure 12.

The human body is usually warmer than its surroundings. After anatural disaster such as an earthquake, search-and-rescue teams useinfrared cameras to locate victims quickly—even underground.

Visible LightThe visible part of the electromagnetic spectrum is light that thehuman eye can see. Each wavelength in the visible spectrum corre-sponds to a specific frequency and has a particular color. Figure 13shows the wavelength and frequency ranges of differentcolors of light in a vacuum.

People use visible light to see, to help keep themsafe, and to communicate with one another. Light enablespeople to read. It is what makes flowers, boxes, signs, andall other objects visible. Automobiles have headlights andtaillights that make night driving safer. Traffic lights com-municate information to drivers about what is expected ofthem—to stop, for example, when the light is red.

What is the visible part of theelectromagnetic spectrum?

Figure 12 A thermogram can beused to diagnose problems in autility line. A When viewed invisible light, the wires all look thesame. B The colors in thethermogram image show that theelectric current in the center wireis not flowing as it should.

Color Wavelength Frequency(nm) (� 1014 Hz)

Red 610–750 4.9–4.0

Orange 590–610 5.1–4.9

Yellow 570–590 5.3–5.1

Green 500–570 6.0–5.3

Blue 450–500 6.7–6.0

Violet 400–450 7.5–6.7

The Visible Spectrum

Figure 13 Each color of lightcorresponds to a different range ofwavelengths. The wavelengths ofvisible light are quite small.Wavelengths of red light, forexample, are about one hundredththe thickness of a human hair.Using Tables As the wavelengthdecreases from the red end of thespectrum to the violet end, whathappens to the frequency?

A B

For: Activity on the greenhouse effect

Visit: PHSchool.com

Web Code: ccc-2182

Infrared RaysUse VisualsFigure 12 Reinforce the idea thatinfrared rays are associated with heat and that they are not part of the visiblespectrum. Tell students that electriccharge dissipates energy in the form ofheat as it flows through a conductor suchas a wire. Tell students that electricalpower transmission lines in goodcondition do not dissipate a lot of heat.Ask, What can you infer from the colors of the pole and the top and bottomwires shown in the thermogram in Figure 12B? (The wires and the pole are at about the same temperature, and theyare not dissipating a lot of heat.) What are some possible reasons why themiddle wire and its insulating resistorare hot? (Student answers may includethat excessive current is flowing through the wire, or that there is damage to thewire causing it to have a much higherresistance than the other wires.) Visual, Logical

FYIA thermogram usually has a key toindicate what temperature each colorcorresponds to.

Visible Light

Because the speed of light in a vacuum is usually given early in any discussion of light, many students fail to realize thatlight has different speeds in differentmaterials. Furthermore, students oftendon’t understand that different colors of light have different speeds in materialssuch as glass. Explain that EM waves,which include many other types ofwaves besides visible light, travel at thespeed of light only when in a vacuum.When EM waves travel through anymedium other than a vacuum, theytravel at speeds less than the speed of light. You may want to use thisinformation to preview Section 18.3,which covers how light interacts with matter. Verbal

L2

L1


Answer to . . .

Figure 13 As wavelength decreases,frequency increases.

The visible part of theelectromagnetic

spectrum is light that people can see.

IPLS

Find links to additional activitiesand have students monitorphenomena that affect Earth and its residents.

For: Activity on electromagneticwaves

Visit: PHSchool.comWeb Code: ccp-2182

Students can learn more aboutelectromagnetic waves online.


Ultraviolet RaysThe wavelengths of ultraviolet rays vary from about 400 nanometersto about 4 nanometers. Ultraviolet radiation has higher frequenciesthan violet light. Ultraviolet rays have applications in health andmedicine, and in agriculture.

In moderation, exposure to ultraviolet rays helps your skin producevitamin D. Vitamin D helps the body absorb calcium from foods toproduce healthy bones and teeth. Excessive exposure can cause sun-burn, wrinkles, and eventually skin cancer. It can also damage your eyes.

Ultraviolet rays are used to kill microorganisms. In heating andcooling systems of large buildings, ultraviolet rays disinfect the air thatflows through the systems. In winter, plant nurseries use ultravioletlights to help plants grow.

X-RaysX-rays have very short wavelengths, from about 12 nanometers toabout 0.005 nanometers. They have higher frequencies than ultravio-let rays. X-rays have high energy and can penetrate matter that lightcannot. X-rays are used in medicine, industry, and transporta-tion to make pictures of the inside of solid objects.

Your teeth and bones absorb X-rays. X-ray photographs show softertissue as dark, highly exposed areas. Bones and teeth appear white. Toomuch exposure to X-rays can kill or damage living tissue.

The lids on aluminum cans are sometimes inspected with X-raysto make sure they are sealed properly. X-rays can be used to identify thecontents of entire truck trailers. Packages and suitcases, such as the onein Figure 14, are X-rayed in search of dangerous contents.

What are X-rays used for?

Evaluating Sunscreen

Procedure1. Insert a black paper strip inside each of two

plastic petri dishes to cover the sides. Place sixultraviolet-detecting beads in each dish. Covereach dish with its lid.

2. On one of the lids, spread a thin layerof sunscreen.

3. Place the dishes in direct sunlight. Record thetime it takes for the beads in each dish tochange color.

Analyze and Conclude1. Comparing and Contrasting Compare

the times the beads in the two dishes tookto change color.

2. Using Models Explain how this lab modelsthe use of sunscreen. What does the colorchange of the beads represent?

3. Predicting How might a sunscreen with ahigher SPF (sun protection factor) affect thetime needed for the beads to change color?

Figure 14 Airport securityscreeners use X-rays to searchbaggage for potentiallydangerous objects.Inferring Why are there darkareas in this X-ray image?

544 Chapter 18

For: Activity on electromagnetic

waves

Visit: PHSchool.com

Web Code: ccp-2182

544 Chapter 18

Ultraviolet Rays

Evaluating Sunscreen

Objective After completing this activity, studentswill be able to • use the SPF of a sunscreen to predict

its effectiveness in blocking ultravioletradiation.

Skills Focus Observing, Measuring

Prep Time 10 minutes

Materials 2 black paper strips, 2 petri dishes, 12 ultraviolet-detectingbeads, sunscreen, clock or watch withsecond hand

Advance Prep Provide each lab groupwith a small vial of the sunscreen labeledwith its SPF.


Safety Make sure that students wearplastic gloves and safety goggles whenapplying sunscreen and clean up whenthey are finished.

Teaching Tips• Make sure that students spread the

sunscreens uniformly.

Expected Outcome The unprotectedbeads change color within five secondsin the presence of sunlight. Dependingon the SPF, sunscreen will delay thischange by as much as two minutes.

Analyze and Conclude1. The beads in the sunscreen-coveredpetri dish took longer to change color.2. The color change representsabsorption of ultraviolet light, whichcauses tanning or burning of skin.Sunscreen absorbs most ultraviolet lightbefore it can affect skin or the beads. 3. A sunscreen with a higher SPF woulddelay the color change for a longer time. Visual, Group

For Enrichment

Have students use the beads to comparethe ultraviolet-protection value of different types of plexiglass, sunglasses,and window glass. Students will discoverthat plexiglass transmits the most ultra-violet light. High-quality sunglassestransmit the least. Kinesthetic, Visual

L3

L2


1. Radio waves, infrared rays, visible light,ultraviolet rays, X-rays, gamma rays2. Radio waves: radio and television,microwave ovens, radar; Infrared rays: sourceof heat, indicator of heat differences, rescuemissions; Visible light: sight, safety, andcommunication; Ultraviolet rays: health,medicine, and agriculture; X-rays: imaging

Section 18.2 Assessment interiors of solid objects in medicine, industry,and transportation; Gamma rays: cancertreatment, imaging the brain, industrialinspection tool3. Radar sends out radio waves and uses thechange in frequency of the reflected waves tocalculate the speed of an object, as deter-mined by the Doppler effect.4. Soft materials show up as dark, highlyexposed areas, while white areas are where X-rays are absorbed.


Reviewing Concepts1. List the kinds of waves included in the

electromagnetic spectrum, from longest toshortest wavelength.

2. Name three uses for each type of wave.

3. How is radar used to determine the speedof a car?

4. How can X-rays make pictures of the inside ofsolid objects?

Critical Thinking5. Comparing and Contrasting How are AM

radio waves similar to FM radio waves? Howare they different?

6. Classifying What type of electromagneticwave are microwaves and radar?

7. Predicting Which do you think will penetratefarther into a block of lead, X-rays or gammarays? Explain your reasoning.

Gamma RaysGamma rays have the shortest wavelengths in the electromagneticspectrum, about 0.005 nanometer or less. They have the highestfrequencies and therefore the most energy and the greatest penetrat-ing ability of all the electromagnetic waves. Exposure to tiny amountsof gamma rays are tolerable, but overexposure can be deadly.

Gamma rays are used in the medical field to kill cancer cellsand make pictures of the brain, and in industrial situations as an inspection tool.

Gamma rays are used in radiation therapy to kill cancer cells with-out harming nearby healthy cells. Gamma rays are also used to makepictures of the human brain, with different levels of brain activity rep-resented by different colors. Four brain scans are shown in Figure 15.

Pipelines are checked with machines that travel on the inside of apipe, taking gamma ray pictures along the entire length. Techniciansexamine the pictures for rusting, cracks, or other signs of damage.


Figure 15 Gamma rays emittedby radioactive tracers in the brainare used to produce color-codedimages. Areas of high activityshow up in red. These imagesshow where the brain is activewhen the patient is (from left toright) looking at something,listening, speaking, and thinkingand speaking. The more involvedthe task, the more parts of thebrain are activated.

Explanatory Writing Write one paragrapheach about three different kinds of electro-magnetic waves that you will encountertoday. Use a single characteristic, such aswavelength or frequency, to describe eachwave. Explain how life might be differentwithout each kind of wave.

Looking Listening Speaking Thinking and Speaking

X-RaysIntegrate BiologyTo determine the structures of proteins,biologists at the Argonne NationalLaboratory Structural Biology Center areshining X-rays onto frozen crystals ofproteins. The X-ray images are capturedby a quick, electronic camera. With thehelp of advanced software this informa-tion is converted into three-dimensionalimages that biologists can study todetermine how the proteins work. Havestudents find out the role that proteinsplay in living cells and have them write a short report. Verbal, Portfolio

Gamma RaysIntegrate Space ScienceSome deep-space objects emit bursts ofgamma rays. These rays travel throughthe empty vacuum of space for millionsof years before reaching Earth. However,Earth’s atmosphere blocks the gammarays and keeps them from reaching thesurface. Because the rays are blocked,astronomers who want to detect andmonitor gamma ray emissions useinstruments that orbit above Earth’satmosphere. Ask students to researchthe cause of deep space gammaemissions and to summarize theirfindings in a short paragraph. Logical, Portfolio

ASSESSEvaluate UnderstandingAsk students chosen at random to listthe general properties of and oneapplication for each electromagneticwave type discussed in the section.

ReteachUse Figure 9 to review the section’s keyconcepts, emphasizing the relationshipbetween the frequency and the wave-length of all electromagnetic waves.


L1

L2

3

L2

L2


5. AM and FM are similar in that they are ameans of coding and transmitting informationon radio waves. They are different becauseAM signals modulate amplitude while FMsignals modulate frequency. AM signals travelfarther because they reflect off particles in theupper atmosphere.6. Radio waves7. Gamma rays should penetrate farther intolead because they have higher-energy photons.

Sample answer: Today I will encountermicrowaves when I cook, infrared rays when Iuse a TV remote control, and ultraviolet lightfrom the sun. These vary in wavelength, withmicrowaves the longest wavelength andultraviolet rays the shortest. If these waves didnot exist, I would have to cook differently, usea different kind of remote control, andperhaps need to use sunscreen less often.

Answer to . . .

Figure 14 Dark areas are regionsthrough which X-rays easily pass.

In medicine, industry,and transportation to

image the inside of solid objects


18.3 Behavior of Light

Reading StrategyMonitoring Your Understanding Copythe flowchart below. As you read, complete itto show how different materials affect light.

Key ConceptsWhat three types ofmaterials affect thebehavior of light?

How does light behavewhen it enters anew medium?

Vocabulary◆ transparent ◆ translucent ◆ opaque ◆ image◆ regular reflection◆ diffuse reflection◆ mirage◆ polarized light◆ scattering

Materials can be

a. ?transparent

wood

and an example of each is

b. ?

c. ? d. ?

What would you see if you were snorkeling in warm ocean watersover a coral reef? You might see fish of bright colors, clown fish, seastars, sponges, and clams. You might see sharks or turtles, and ofcourse, coral. But why can you see these animals so clearly? Why canyou see the reef through the water but not, for example, through thebottom of the boat that brought you to the reef?

Light and MaterialsWithout light, nothing is visible. When you look at the reef animals,what you are really seeing is light.You can see the reef through the water,because light passes through the water between the reef and your eyes.But you can’t see the reef through the bottom of the boat because lightdoesn’t pass through the boat.

How light behaves when it strikes an object depends on many fac-tors, including the material the object is made of. Materials can

be transparent, translucent, or opaque. Each type of material affectsthe behavior of light in different ways.

A material through which you can see clearly is transparent.A transparent material transmits light, which means it allowsmost of the light that strikes it to pass through it. For example,the water where the fish and coral in Figure 16 live is transpar-

ent. While riding on a bus, you can see buildings and treesoutside because the bus windows are transparent.

546 Chapter 18

Figure 16 Water is transparent.You can see through it. Thatcharacteristic makes it possible tophotograph these fish and otheranimals living in the ocean.

546 Chapter 18

FOCUS

Objectives18.3.1 Classify materials as

transparent, translucent, oropaque to visible light.

18.3.2 Describe what happens whenlight is reflected, refracted,polarized, or scattered.

Build VocabularyConcept Map Have students constructa concept map of the vocabulary termsused in this section. Instruct students toplace the vocabulary terms in ovals andconnect the ovals with lines on whichlinking words are placed. Studentsshould place the main concept (Behaviorof Light) at the top or the center. As theymove away from the main concept, thetopics should become more specific.

Reading Strategya. translucent b. opaque c. glass orwater d. frosted glass

INSTRUCT

Light and MaterialsBuild Reading LiteracyAnticipation Guide Refer to page 388D in Chapter 13, whichprovides the guidelines for an anticipation guide.

Ask students which statements are true:1. All of the light striking an object isabsorbed by that object. (False) 2. It ispossible to see an object in a room thatis completely dark using only your eyes.(False) 3. The cover of your book doesnot reflect light. (False)

Have students read the text on pp. 546 and 547, then revisit the three questions. Verbal

FYIA material need not be transparent oropaque to all forms of electromagneticradiation. For example, wood is opaqueto visible light, but not to all electro-magnetic waves. Microwaves and radiowaves can pass through wood.

L2

2

L2

L2

Reading Focus

1

Section 18.3

Print• Laboratory Manual, Investigation 18B• Reading and Study Workbook With

Math Support, Section 18.3• Transparencies, Section 18.3

Technology• Interactive Textbook, Section 18.3• Presentation Pro CD-ROM, Section 18.3

Section Resources


If you can see through a material, but the objects you see throughit do not look clear or distinct, then the material is translucent (transLOO sunt). A translucent material scatters light. The soaps in Figure 17Aare translucent. When you look into a room through a frosted glassdoor, you can make out shapes of people and objects, but the shapesare fuzzy and lack detail.

Most materials are opaque (oh PAYK). An opaque material eitherabsorbs or reflects all of the light that strikes it. The fruit in Figure 17Bis opaque. An opaque object does not allow any light to pass throughit. You can’t see through a wooden table or a metal desk. Wood andmetal are examples of opaque materials.

Interactions of LightWhen light encounters matter, some or all of the energy in the light canbe transferred to the matter. And just as light can affect matter, mattercan affect light. When light strikes a new medium, the light canbe reflected, absorbed, or transmitted. When light is transmitted, itcan be refracted, polarized, or scattered.

Reflection When you look in a mirror, you see a clear image ofyourself. An image is a copy of an object formed by reflected (orrefracted) waves of light. Similarly, when you look at a still lake, youcan see a sharp reflected image of the far shore. But what happens to thereflected image in the lake if the wind suddenly gusts, causing ripples inthe surface of the water? The image is blurred, or fuzzy-looking. Whenlight reflects from a smooth surface, you see a clear, sharp image. Whenlight reflects from a rough surface, you see a blurred reflected image orno image at all.

Regular reflection occurs when parallel light waves strike a sur-face and reflect all in the same direction. Regular reflection happenswhen light hits a smooth, polished surface, like a mirror or the surfaceof a still body of water such as in Figure 18.

Diffuse reflection occurs when parallel light waves strike a rough,uneven surface, and reflect in many different directions. If you couldlook at this page of your book through a microscope, you would seethat the paper has a rough surface. The rough surface causes diffusereflection of the light that shines on it.

Figure 18 Almost all objectsreflect light. A In regularreflection, a smooth surfacereflects a clear image becauseparallel light waves reflect all inthe same direction. B In diffusereflection, parallel light wavesreflect in many directions.

Figure 17 When light strikes anew medium, it can be reflected,absorbed, refracted, polarized, orscattered. A The translucent barsof soap scatter light, making thesoaps and what you can seethrough them appear fuzzy. B You cannot see through thefruit because opaque materials donot transmit any light.

B

A

Regular reflectionA Diffuse reflectionB

Interactions of Light

Although most students accept thatmirrors reflect light, some studentsreject the idea that ordinary objects also reflect light. This misconception isrelated to another one that manystudents have, namely that their eyes donot receive light when observing anobject. Probe student misconceptionsabout vision by asking them to explainwhat makes it possible for people to see.You may discover a range of miscon-ceptions, including the idea that lightfills space (as expressed by saying, “theroom is full of light”) and the idea that itis the action of people’s eyes that makesobjects observable. Counter miscon-ceptions using a careful explanation ofthe connections among reflected light,emitted light, and vision.Verbal

Reflected Light and VisionPurpose Students observe the role ofreflected light in vision perception.

Materials medium-sized wooden orcardboard box with its interior lined withwhite paper or painted bright white. Boxtop must be removable or hinged, andhave a small hole in its center.

Procedure Show students the boxwith its top on under normal roomlighting conditions. Have students usetheir observations of the box (and thehole) to predict the box’s interior color.

Expected Outcome Most studentswill not predict that the box’s interior iswhite. Show students the white interiorof the box. Explain that whateveramount of light enters the box, it iseventually absorbed after numerousreflections off the white interior surfaces.Because no light escapes the box andtravels to the students’ eyes, it is notpossible to discern the interior color ofthe box. Visual, Logical

L2

L2



Increase Word ExposureAs students read through the section in class,post the section’s nine vocabulary terms on a“word wall.” Carefully define each word when it first appears. Then, ask for student help inorganizing the words into groups based on their

science meanings. Use the word wall as aninteractive tool to help students become familiarwith the terms. Make frequent references to thewords and assign them in homework or anactivity to validate their importance.


Vertical polarizingfilter

Horizontal waveis blocked.

Vertical wave passesthrough filter.

Vertical waveis blocked.

Horizontal polarizingfilter

Refraction A light wave can refract, or bend, when it passes at anangle from one medium into another. You can easily observe twocommon effects of refraction when light travels from air into water.Refraction makes underwater objects appear closer and larger than theyreally are. Refraction can also make an object, such as a skewer, appearto break at the surface of the water, as shown in Figure 19.

Refraction can also sometimes cause a mirage. A mirage is a false ordistorted image. Mirages occur because light travels faster in hot air thanin cooler, denser air. On a sunny day, air tends to be hotter just abovethe surface of a road than higher up. Normally, light travels from the sunall the way to the ground before being reflected. But on a hot day, lightis gradually refracted as it moves into layers of hotter and hotter air. Thisgradual refraction causes some of the light to follow a curved path, ratherthan a straight path to the ground. Light that reaches your eyes aftertraveling in this manner can look as if it was reflected from a layer ofwater. Mirages also form this way above the hot sand in deserts.

Polarization Light with waves that vibrate in only one plane ispolarized light. Polarizing filters transmit light waves that vibrate inthis way. Look at Figure 20. Unpolarized light vibrates in all directions.A vertical polarizing filter stops waves vibrating on a horizontal plane.Waves vibrating on a vertical plane pass through. A horizontal polariz-ing filter then blocks the waves vibrating on a vertical plane. Tounderstand how a polarizing filter works, think of a light wave as beinglike a postcard that you push through a mail slot in a door. If you holdthe postcard so that it lines up with the mail slot, then you can easilypush it through. But if the postcard is at an angle to the mail slot, it hastrouble passing through. In the same way, a polarizing filter blockswaves with electric fields vibrating in one direction.

What is a mirage?

Figure 20 This simplified modelshows how polarizing filtersbehave. A vertical polarizing filterblocks light that is horizontallypolarized. Applying Concepts Whatwould happen if you looked atlight through a horizontallypolarizing filter and a verticallypolarizing filter at the same time?

548 Chapter 18

Figure 19 Light refracts, orbends, when it moves from onemedium to another. Because thelight bends, the image you seeappears to be bent as well. Relating Cause and Effect Whydo the underwater parts of theskewers appear to be closer toyou than the parts above water?

548 Chapter 18

Build Science SkillsObserving

Purpose Students observe the effect of polarizing filters.

Materials 2 polarizing filters, light source


Procedure Have students work in agroup. One group member should holdone filter up to light coming through awindow and observe the result. Ask,Why is the light dimmer after passingthrough one of the filters? (The filterhas absorbed some of the light.) Haveanother group member align both filtersin the same direction and then view thelight from the window. Ask, How doesthe brightness compare with whatyou saw when the light passedthrough a single filter? (The light isdimmer after passing through both filters.)Have a third group member try toarrange the filters in such a way that nolight is transmitted through the filters.

Safety Caution students to not lookdirectly at the sun.

Expected Outcome Students willobserve how the orientation of the twofilters determines the amount of lightthat passes through. They will see thatwhen the two filters are oriented at 90˚to one another, no light is transmitted.Kinesthetic, Group

Use VisualsFigure 20 Point out to students thatFigure 20 represents how a verticalpolarizing filter blocks light that ishorizontally polarized. Tell students thatthe glare from horizontal surfaces suchas roads and automobile hoods tends tobe horizontally polarized. Then ask, Inwhat direction should the polarizinglenses in a pair of polarizing sun-glasses be aligned? (To block thehorizontally polarized glare, the lensesshould be positioned with their polarizingslits aligned vertically.)

FYIAn actual Polaroid polarizing filterconsists of long chain molecules that arestretched preferentially in one direction.Light with an electric field parallel to thechain molecules is absorbed.

L1

L2


Rainbows A rainbow is a series of concentriccolored arcs that form when light from the sunpasses through air containing water drops,usually from rain or fog. The rainbow is visiblein the direction opposite to that of the sun.The different colors are caused by therefraction and internal reflection of light raysthat enter the water drop. Because sunlight ismade of light of varying frequencies, thesedifferent frequencies refract at different angles

in the water drop. The different frequencies(and colors) emerge from the drop and formbands of color in the sky. One edge of therainbow is red (the wavelength that refractsthe least in the water drop) and the otheredge is violet (the wavelength that refracts themost in the water drop). The other colors ofthe visible spectrum fill in the region betweenred and violet.

Facts and Figures


Reviewing Concepts1. Explain the differences among opaque,

transparent, and translucent materials. Nametwo objects made from each type of material.

2. List and explain three things that canhappen to a light wave when it enters anew medium.

3. What is the difference between diffusereflection and regular reflection?

4. What happens to light that passes through ahorizontal polarizing filter?

Critical Thinking5. Predicting A black car reflects much less light

than a white car. Which car’s surface will bewarmer after 1 hour of sunshine? Explain.

6. Formulating Hypotheses A mountainclimber finds that her sunglasses are notblocking glare from a vertical rock wall. Whatcan you hypothesize about the polarizingfilters in her sunglasses?

7. Applying Concepts On a foggy night, youcan see a car’s headlight beams but you maynot be able to see the car itself. Explain why.

Light reflecting from a nonmetallic flat surface, such as a windowor the surface of a lake, can become polarized. When sunlight reflectsfrom a horizontal surface, horizontally polarized light reflects morestrongly than the rest of the sunlight. This reflection produces glare. Toblock the glare, polarized sunglasses have vertically polarized filters,which block the horizontally polarized light.

Scattering Earth’s atmosphere contains many mol-ecules and other tiny particles. These particles can scattersunlight. Scattering means that light is redirected as itpasses through a medium. Look at Figure 21. A scatter-ing effect reddens the sun at sunset and sunrise. Most ofthe particles in the atmosphere are very small. Small par-ticles scatter shorter-wavelength blue light more thanlight of longer wavelengths. The sunlight encountersmore of the molecules and tiny particles that scatter theshorter-wavelength colors. By the time the sunlightreaches your eyes, most of the blue and even some of the green andyellow have been scattered. Most of what remains for your eyes todetect are the longer wavelengths of light, orange and red.

When the sun is high in the sky, its light travels a shorter distancethrough Earth’s atmosphere. It scatters blue light in all directions muchmore than other colors of light. Scattering explains why the sky appearsblue on a sunny day, even though air itself is colorless.


Mechanical Waves Review the behaviorsof mechanical waves discussed in Section17.2, such as reflection and refraction.Compare them with the behaviors of light.

Figure 21 The lower the sun is onthe horizon, the more of theatmosphere the light travelsthrough before it reaches Earth’ssurface. In certain weatherconditions, the blue, green, andyellow wavelengths of sunlightare heavily scattered. What’s leftto enjoy are the beautiful redsand oranges of sunrise and sunset.

ASSESSEvaluate UnderstandingAsk students to identify objects orlocations in the classroom that areexamples of transparency, translucency,opaqueness, regular reflection, anddiffuse reflection. Have students explainwhy their example is correct.

ReteachUse Figures 16 to 21 as examples thatillustrate the key concepts of the section.

Reflection: In mechanical waves, theenergy of the reflected wave is less thanthe energy of the original wave. Thesame is true of light waves because somelight will be absorbed or transmitted.Refraction: In mechanical waves,refraction occurs because one side of awave front moves more slowly than theother side when the wave enters amedium at an angle. Light waves alsoare refracted because they change speedas they move from one medium intoanother.


L1

L2

3


Answer to . . .

Figure 19 Refraction of light fromthe skewers makes them appear largerthan they really are.

Figure 20 The two filters would blockall of the light.

A mirage is a false ordistorted image.

3. Diffuse reflection: parallel light waves arereflected in many directions; Regular reflection:reflected parallel light waves are still parallel. 4. Only horizontally polarized light comes outthe other side of the filter; all of the rest ofthe light is blocked by the filter.5. The black car absorbs much more of thelight striking it, so it will be warmer than thewhite car.

6. The sunglasses have vertically polarizedfilters that transmit vertically polarized glarereflected from the vertical rock wall. 7. Particles of water in the fog scatter thelight from the car’s headlights, creating anilluminated “beam” of light that is visible infront of a car. If the car is coming around acorner, the headlight beam is visible beforethe car is seen.


1. Transparent materials (glass window,water) transmit light. Translucent materials(soaps, frosted glass) scatter light that passes through. Opaque materials (woodtable, metal desk) absorb or reflect allincident light.2. The light can be reflected (bounce off), be absorbed (enter but not exit), or betransmitted (pass through).


18.4 Color

Key ConceptsHow does a prismseparate white light?

What determines the colorof an object?

What are the primarycolors of light?

What are the primarycolors of pigments?

Vocabulary◆ dispersion◆ primary colors◆ secondary color◆ complementary

colors of light◆ pigment◆ complementary

colors of pigments

Have you ever zoomed in on a color photograph displayed on a com-puter screen? If you have, you’ve seen that the photograph is made up ofmany tiny squares, called pixels, as shown in Figure 22. Your computerscreen might be set to display 256 colors, thousands of colors, or evenmillions of colors. All of these colors are generated using various com-binations of only three colors of light. If you were to print the samephotograph, your printer would also use only three colors, plus black, tocreate the image out of tiny dots of ink. The three colors the computeruses are different from the three the printer uses. How can that be?

Red

Green

White

b. ?

c. ?

a. ?

d. ?

Reading StrategyVenn Diagram Copy the Venn diagrambelow. After you read, label the diagram formixing primary colors of light. Make a similardiagram for mixing primary colors of pigments.

Figure 22 This student is lookingat many colors on his computerscreen. What he is actually seeing,however, are combinations ofonly three colors of light.

550 Chapter 18

550 Chapter 18

FOCUS

Objectives18.4.1 Explain how a prism disperses

white light into different colors.18.4.2 Analyze factors that determine

the color of an object.18.4.3 Distinguish among primary,

secondary, and complementarycolors of light and of pigments.

Build VocabularyVocabulary Knowledge Rating Chart Have students construct achart with four columns labeled Term,Can Define or Use It, Have Heard or SeenIt, and Don’t Know. Have students copythe six terms into the first column andrate their term knowledge by putting acheck in one of the other columns. Askhow many students actually know eachterm. Have them share their knowledge.Ask focused questions to help studentspredict text content based on the term,thus giving students a purpose forreading. After students have read thesection, have them re-rate themselves.

Reading Strategya. Blue b. Magenta c. Cyan d. YellowFor primary colors of pigments:

L2

L2

Reading Focus

1

Section 18.4

Print• Laboratory Manual, Investigation 18A• Reading and Study Workbook With


Technology• Interactive Textbook, Section 18.4• Presentation Pro CD-ROM, Section 18.4• Go Online, NSTA SciLinks, Color

Section Resources

Magenta Cyan

Yellow

Red Green

Black

Blue


Separating White Light Into ColorsIn 1666, the English physicist Isaac Newton investigated the visiblespectrum. First, he used a glass prism to produce a visible spectrumfrom sunlight. With screens, he then blocked all colors of light exceptblue. Next, he placed a second prism where the blue light was visible.The second prism refracted the blue light but had no further effect onthe color. Newton’s experiments showed that white sunlight is made upof all the colors of the visible spectrum.

How does a prism separate white light into a visible spectrum?As white light passes through a prism, shorter wavelengths

refract more than longer wavelengths, and the colors separate. Lookat Figure 23A. When red light, with its longer wavelength, enters aglass prism, it slows down the least of all the colors, and so is bent theleast. Violet light is bent the most. The process in which white lightseparates into colors is called dispersion.

A rainbow gives a beautiful example of dispersion. Droplets of waterin the air act like prisms. They separate sunlight into the spectrum.When light enters a raindrop, it slows down and refracts. Then it reflectsoff the far inner surface of the raindrop. It refracts again as it exits theraindrop, speeds up, and travels back toward the source of the light.

The Colors of ObjectsWould it surprise you to learn that an object of any color does not havea definite color? An object’s color is the color of light that reaches youreye when you look at the object. The color of any object dependson what the object is made of and on the color of light that strikes theobject. Sunlight contains all the colors of the visible spectrum. Butwhen you look, for example, at a red car in sunlight, the red paintreflects mostly red light. Most of the other colors in white light areabsorbed at the surface of the paint.

What process causes a rainbow?

Figure 23 White light isdispersed by prisms and waterdroplets. A When white lightpasses through a prism, theshorter wavelengths are bentmore than the longerwavelengths. The colors areseparated. When red light entersthe second prism, it is refractedbut the color does not change. B Water droplets separate thecolors of sunlight, producinga rainbow.

A

B

For: Links on color


Web Code: ccn-2184

INSTRUCT

Separating WhiteLight Into ColorsUse VisualsFigure 23 Review refraction withstudents. Ask, After leaving the prism,how does the path of red lightcompare to the path of blue light?(Both colors travel in a straight line. The redlight travels at a different angle because it refracts less when passing through theprism.) If red and blue light refractedthe same amount, would they beseparated by the prism? (No) What isthe purpose of the white card with thehole? (It blocks all of the colors except red.)What happens to the red light at thesecond prism? (The red light changesdirection but is not separated into newcolors). What can you conclude aboutthe nature of red light and white light?(Red light is more fundamental than whitelight because it cannot be separated intoother colors.)Visual, Logical

The Colors of ObjectsBuild Science SkillsControlling Variables Help studentssee the importance of changing onevariable at a time. If you look at a redobject in white light and then look at ablue object in green light, two variableschange, so you cannot draw validconclusions. By comparing an object(with one color) under two differentcolors of light, only one variablechanges, so valid conclusions can be drawn. Logical, Visual

L2

L1

2



Think-Pair-ShareHave students work in pairs to think ofsituations in which the color of an objectchanges because of the light shining on it.Examples include a building at sunset or a shirtviewed under colored lights at a dance.

Strengthen discussion skills by having studentsshare their examples with the class. Encouragestudents to refer to Figure 24 for help inexplaining the colors seen under differentlighting conditions.

Answer to . . .

A rainbow is caused by dispersion.

Download a worksheet on color forstudents to complete, and findadditional teacher support fromNSTA SciLinks.


552 Chapter 18

What happens if you change the color of the light shining on anobject? Look at Figure 24, which shows the same stack of flower potsin several different colors of light. The pots appear to be differentcolors when viewed in different colors of light. For example, the red potlooks black when viewed in blue light because the red plastic absorbsall of the light striking it. No red light reaches the object, so none canbe reflected from it.

Mixing Colors of LightFigure 25 shows how equal amounts of three colors of light—red,green, and blue—combine to produce white light. Primary colors arethree specific colors that can be combined in varying amounts tocreate all possible colors. The primary colors of light are red,green, and blue.

When red light strikes a white surface, red light is reflected.Similarly, when blue light strikes a white surface, blue light is reflected.What happens if both red light and blue light strike a white surface?Both colors are reflected and the two colors add together to make athird color, magenta. When colors of light are mixed together, thecolors add together to form a new color.

The secondary colors of light are cyan, yellow, and magenta. Eachsecondary color of light is a combination of two primary colors.Therefore, if you add a primary color to the proper secondary color,you will get white light. Any two colors of light that combine to formwhite light are complementary colors of light. A complementary colorpair is a combination of one primary color and one secondary color.Blue and yellow are complementary colors of light, as are red and cyan,and green and magenta.

How is a secondary color of light formed?

Figure 24 Under white light, thepots appear white, green, yellow,red, and blue. Observing Howdoes the red pot appear underred, green, and blue light?

Figure 25 The three primarycolors of light are red, green, andblue. When any two primarycolors combine, a secondary coloris formed. Observing What colorof light is produced when allthree primary colors combine inequal amounts?

Red

BlueGreen

MagentaYellow

Cyan

552 Chapter 18

Mixing Colors of Light

Many students do not understand thedifference between mixing colored lightsand mixing pigments. Point out thatwhen two beams of differently coloredlight overlap, colors are added. This resultis in contrast to the more familiarexperience of subtracting colors of lightby mixing paints or inks that containdifferent light-absorbing pigments. Visual

Build Reading LiteracyCompare and Contrast Refer to page 226D in Chapter 8, whichprovides the guidelines for comparingand contrasting.

Students can avoid confusing light andpigments by studying similarities anddifferences in Figure 25 and Figure 26.Have students complete a compare-contrast table. Ask, How are the figuressimilar? (Both have primary colors,secondary colors, and include red, green,blue, cyan, yellow, and magenta.) Howare the figures different? (The threeprimary colors form white in one figureand black in the other. Red, blue, andgreen are primary colors of light butsecondary colors of pigment. Cyan, yellow,and magenta are secondary colors of lightbut primary colors of pigment.)Visual

L1

L2


Wavelength, Frequency, and Color It isbetter to think of color in terms of frequencythan in terms of wavelength. The frequency of a wave is determined by the frequency of the source of the wave. When a light waveenters a new medium, the wavelength andspeed of the wave change, but the frequencystays the same.

From an energy standpoint, a photon has aspecific energy equal to Planck’s constantmultiplied by the frequency of the wave. Thus,a photon is defined by frequency, not bywavelength. When a photon enters a newmedium, it slows down and its wavelengthchanges, but its energy (and therefore itsfrequency) remains unchanged.

Facts and Figures


Reviewing Concepts1. Explain how a prism separates white light

into the colors of the spectrum.

2. What determines the color of an object?

3. What three colors of light can combine toform any other color?

4. What three colors of pigments cancombine to form any other color?

5. Explain how the process of dispersion of lightforms a rainbow.

6. What are pigments? Explain how differentpigments affect light.

Critical Thinking7. Predicting What color would a blue wall

appear under green light?

8. Applying Concepts Why does combiningequal amounts of cyan, yellow, and magenta paints form black?

Mixing PigmentsPaints, inks, photographs, and dyes get their colorsfrom pigments. A pigment is a material thatabsorbs some colors of light and reflects othercolors. Stone Age cave paintings were made withnatural pigments from colored earth and clay. Overthe centuries, natural pigments have been obtainedfrom many sources, including metal oxide com-pounds, minerals, plants, and animals. Today’sartists use paints made from natural pigments aswell as from synthetic, or manufactured, pigments.

The primary colors of pigments are cyan,yellow, and magenta. Perhaps you have noticed thatcolor printers and photocopiers use these threecolors, plus black. You can mix varying amounts ofthese primary pigment colors to make almost anyother color. Each pigment reflects one or more colors. As pigments aremixed together, more colors are absorbed and fewer colors are reflected.When two or more pigments are mixed together, the colors absorbed byeach pigment are subtracted out of the light that strikes the mixture.

Look at Figure 26. The light filters absorb light in much the sameway pigments do. When cyan and magenta are combined, blue isformed. Cyan and yellow combine to form green. Yellow and magentacombine to form red. The secondary colors of pigments are red, green,and blue. Any two colors of pigments that combine to make blackpigment are complementary colors of pigments.


Figure 26 The three primarycolors of pigments are cyan,yellow, and magenta. When thethree primary colors of pigmentsare combined, the secondarycolors of pigments are formed. Interpreting Diagrams Whichcolors of pigments combine tomake black?

Explain a Concept Write a letter to a friendwho is not in class with you. Explain how anobject gets its color. Give evidence and useexamples to support your explanation.

Magenta

Yellow

Cyan

Green

Blue

Red

Mixing Pigments

Overlapping FiltersPurpose Students investigate howcolors combine by addition andsubtraction.

Materials 2 overhead projectors; red, green, and blue acetate filters

Procedure On the surface of anoverhead projector, overlap red andgreen filters. Have students describewhat they see projected on the screen.Ask them to predict what will happenwhen red and blue or green and bluefilters overlap.

Expected Outcome Each filtersubtracts all colors except one, so twooverlapping filters subtract all colors toproduce black. Visual, Group

ASSESSEvaluate UnderstandingHave students invent stage-lightingscenarios in which they describeclothing seen under a particular color oflight. Encourage students to brainstormwhat the clothing colors might be whenviewed in white light.

ReteachCompare Figures 25 and 26 to reviewhow colors of light are different fromcolors of pigments.

Student letters should discuss how coloris reflected by an object and how thecolor depends on the color(s) of lightshining on the object.


L1

L2

3

L2


6. A pigment is a material that absorbs somecolors and reflects others; each pigmentreflects different colors. 7. A blue wall reflects blue light, so undergreen light the blue wall will appear black.8. Cyan, yellow, and magenta paints combinein equal amounts to form black paint becausethese are the three primary colors of pigments.


1. Each wavelength refracts a differentamount, separating the colors in white light. 2. The color depends on what the object ismade of and the color(s) shining on the object. 3. Red, green, and blue4. Cyan, yellow, magenta5. Light refracts as it passes in and out ofwater droplets, separating the colors of the rainbow.

Answer to . . .

Figure 24 In red light, the red pot isred; in blue light or green light, the redpot looks black.

Figure 25 White light

Figure 26 Cyan � magenta �yellow; cyan � red; yellow � blue;magenta � green

It is formed when twoprimary colors combine.


Scientific methods cannot prove that a paintingis genuine, but they can prove that it is a forgery.Careful examination may reveal aspects of thepainting—the artist’s methods or the materialsused—that come from a time period later thanthe supposed date of the painting.

The simplest techniques used to examinepaintings are those involving visible light.Looking at a cross section of a paint samplewith a microscope or hand lens shows how theartist built up the layers of paint.

Scientists are able to examine the structureof the paint pigment in detail using high-powered microscopes. The structure can revealwhether the pigment is natural or has beenmade synthetically. Because synthetic paintpigments were not developed until the 1800s,the presence of synthetic pigment indicatesthe earliest date for the painting.

Interpreting visible lightThis is the part of the electromagneticspectrum to which human eyes are sensitive.Paint pigments reflect some parts of thevisible spectrum and absorb others, so theeye perceives them as colored.

New Light on Old ArtHow does an expert know if a painting is by a famousartist, or is a fake? Scientific methods use electromagneticradiation—visible light, ultraviolet, infrared, and X-rays.

Examining samplesUsing scalpels and tweezers,experts take tiny samplesfrom a painting to examinethe painted surface.

Paintpigment

Scalpel

Tweezers

Microscope

Fibers

VISIBLE LIGHT

554 Chapter 18

554 Chapter 18

New Light on Old ArtBackgroundWhen a painting is suspected of being a forgery, scientists analyze it using avariety of noninvasive techniques suchas optical microscopy, scanning electronmicroscopy (SEM), X-ray radiography,X-ray fluorescence (XRF), and infraredphotography. In optical microscopy,light from the painting passes through a series of lenses in a microscope toproduce a magnified image of the paint-ing’s surface. In X-ray radiography, thepainting is bombarded with X-rayradiation. Areas that contain high-atomic-weight pigments such as whitelead and lead-tin yellow appear light inthe resulting image. Cracks and areas oflow-atomic-weight pigments absorbfewer X-rays and appear dark.

X-ray fluorescence (XRF) is able todetermine the types of elements presentby bombarding the painting with X-rays.Instead of being scattered, the X-rays areabsorbed. The energy that is absorbedresults in the emission of X-rays from thepaint. The frequency of the emitted X-ray identifies the elements in the paint.

Infrared photography is used to revealthe presence of charcoal or graphitedrawings beneath the paint layers. Thedrawings are revealed because theinfrared radiation is absorbed by carbonin the drawing, but not by othersubstances in the paint.

L2

Under the microscopeHigh magnifications (100 to 500 times)show the physical structure of pigment

particles. Here, the particles in the sampleof synthetic ultramarine are different from

those in the natural sample. Syntheticultramarine was introduced in 1828, sopaintings using this pigment could nothave been made earlier than that year.

Natural materialsBefore the 1800s, mostpigments used by artistswere from naturalsources. Many, includingyellow lake, wereobtained from plants.Some, such as carmine orcochineal, were of animalorigin. Others, likeultramarine, came from minerals.

Yellow lakepigment

Carminepigment

Ultramarinepigment

COCHINEAL BEETLES

BUCKTHORNBERRIES

Synthetic pigment hasround, fine particles

Natural pigment hasrough, crystalline particles

Powderedpigment

Examining the Paint SurfaceIdentifying the pigments used in a paintingusually requires taking one or more tinysamples from the painting. This samplingmust be done with extreme care.

Lapis lazuli was groundto a powder to makeultramarine pigment.


Build Science SkillsObservingHave students find out about somefamous art forgeries by using theInternet or the library. Have them printor photocopy the forgeries. Then, havestudents look for a photograph of theoriginal painting in art books or anencyclopedia. Have the class comparethe two paintings. Ask, Can you see anydifferences between the forgery andthe real painting? (The differences willdepend upon the paintings chosen.) Whattests could be done to determine ifthe painting is a forgery? (Possibleanswers may include, examine a crosssection of a paint sample under amicroscope or hand lens; or examine anintact painting with ultraviolet light,infrared, and X-rays.)Visual, Portfolio

L2



Rather than removing samples of the paintlayer, non-invasive methods examine an intactpainting by using the nonvisible regions of theelectromagnetic spectrum. Ultraviolet, infrared,and X-rays can reveal many secrets that areinvisible to the human eye.

Ultraviolet rays are best at showing surfacefeatures. The varnish that is the top layer ofmost paintings will fluoresce when exposed toultraviolet. This fluorescence shows whetherthe original varnish has been disturbed.

Infrared rays can penetrate the layers ofpaint, so infrared imaging can be used todetect charcoal sketches and other images thatare often hidden beneath the painted surface.

X-rays are used to look through a painting.X-rays are absorbed by dense materials and passthrough others. Pigments that include metalatoms, such as white lead, show up clearly, as dometal objects used in the painting’sconstruction. One forger was caught when X-rays revealed a machine-made nail in thewooden panel under a painting that wassupposedly painted in the 1500s! Machine-madenails were not manufactured until the 1800s.

Looking Beneath the Paint Surface

Art forgery detectionA technician operates an infraredscanner. He is examining a copyof a painting by the Germanpainter Lucas Cranach the Elder(1472–1553).

Interpreting ultraviolet radiation Although transparent to visible light, the varnishlayer on the surface of most paintings can be seenusing ultraviolet rays. In this copy of Cranach’spainting, the ultraviolet makes the varnish layerfluoresce. Dark regions show areas that have beenretouched or painted over.

Darker areashave beenretouched orpainted over.

ULTRAVIOLET RAYS

556 Chapter 18

556 Chapter 18

(continued)

Build Science SkillsComparing and Contrasting

Purpose Students compare and contrast the properties of paints.

Materials acrylic, watercolor, and oilpaints of the same or similar color;several small artist paint brushes;watercolor paper; hand lens; several flat,wooden sticks

Class Time 10 minutes, then 10minutes after one week

Procedure Have students paint a strip of each type of paint on both thewatercolor paper and the flat, woodenstick. Make sure that students label eachstrip with the type of paint used. Placethe strips in an area where they willremain undisturbed. After one week,have students observe all of the paintstrips and record their observations.

Safety Have students wear lab apronsand disposable gloves.

Expected Outcome Students will beable to observe the characteristics ofeach paint type. Kinesthetic, Group

L2

� Choose one of these painting styles toresearch: impressionism, surrealism, pointillism,or op art. Prepare an oral report to share withthe class, including anexplanation of the paintingstyle, how light and color areused in the style, and threesamples of the painting style.

� Take a Discovery ChannelVideo Field Trip by watching“Finding the Fakes.”

Going Further

Interpreting X-rays X-rays can reveal the creativeprocess at work. In this portrait ofPope Julius II, painted by Raphaelin 1511–1512, the X-ray imageshows a pattern of crossed keys onthe wall behind the seated figure.The keys do not appear in visiblelight. Because oil paints are opaqueto visible light, the artist probablysimply painted over the keys.

Horizontal woodenbattens used tostrengthen the panel

Infrared rays penetratelayers of paint, revealing an

underlying self-portrait.

Merry musician inthe final painting

Interpreting infraredimagingThe penetrating power ofinfrared most often uncoverspreliminary sketches. Butinfrared imaging can alsoreveal surprising changes inthe development of a workof art. In this self-portrait byJudith Leyster about1630, the infrared imagereveals that the artistoriginally included a portraitof herself on the easel. Butlater she changed theportrait to a musician.

Part of theoverpainted pattern

of crossed keys

INFRARED RAYS

X-RAYS

Video Field Trip


Going FurtherStudent reports should contain relevantexamples of the selected style. Briefexplanations of each style follow.• Perspective The art of picturing

objects or a scene in such a way as to show them as they appear to theeye with reference to relative distanceor depth.

• Impressionism A theory and schoolof painting exemplified by ClaudeMonet, Camille Pissarro, and AlfredSisley, whose aim was to capture amomentary glimpse of a subject,especially by reproducing thechanging effects of light. They appliedpaint to a canvas in short strokes ofpure color.

• Surrealism A movement in art andliterature in which an attempt is madeto portray or interrupt the workings ofthe unconscious mind as manifestedin dreams.

• Pointillism The method of paintingof certain French impressionists inwhich a white background is system-atically covered with tiny points ofpure color that blend together whenseen from a distance, producing aluminous effect.

• Op Art A style of abstract paintingutilizing geometric patterns or figuresto create various optical effects, suchas the illusion of movement.Visual, Verbal


After students have viewed the Video Field Trip, askthem the following questions: Why are pigmentparticles in older paintings different from thepigment particles in modern paintings?(Pigments ground by hand contained larger particlesthat varied in size. Today, machines use rollers thatproduce smaller, more uniform particles.) Whatsources were used for pigments in old

paintings? (Rocks, minerals, and plants) How canforensic scientists tell if an old painting is aforgery? (Student answers may include, pigmentsfound in the painting were not used in the artist’slifetime; brush strokes were not typical of the artist; orthe canvas was not made of material used in thattime period.) Why is it easier to prove that apainting is a forgery than to prove it is authen-tic? (Even if the pigments, style of brush strokes, andcanvas material all match, the painting could still bethe work of someone of the artist’s own time or thework of a skilled forger.)

Video Field Trip

Finding the Fakes


558 Chapter 18

18.5 Sources of Light

As sunlight fades toward the end of the day, objects around youbecome less and less visible. When the sun has completely set, you canno longer see your surroundings. Objects are invisible in the darkbecause no light is available to reflect off them. But some things, suchas flashlights and fireflies, produce their own light. Objects that giveoff their own light are luminous. The sun is luminous, as are alllight sources.

Common light sources include incandescent, fluorescent,laser, neon, tungsten-halogen, and sodium-vapor bulbs. Each typeof bulb produces light in a different way.

Incandescent LightThe light produced when an object gets hot enough to glow isincandescent. Figure 27 shows an incandescent light bulb. Inside, youcan see the filament, a thin coil of wire stretched between two thickerwires. When electrons flow through the filament of an incan-descent bulb, the filament gets hot and emits light.

The filaments in incandescent light bulbs are made of a substancecalled tungsten. Incandescent light bulbs are filled with a mixture ofnitrogen gas and argon gas at very low pressure. These gases do notreact with the filament as oxygen would, and so the filament lasts longer.Incandescent bulbs give off most of their energy as heat, not light.

Figure 27 An incandescent bulbcontains a filament. As electronsflow through the filament, thefilament gets hot and emits light.Formulating Hypotheses Whyis a 100-watt bulb generallybrighter than a 75-watt bulb?

Reading StrategySequencing Copy and complete theflowchart below. As you read, pick two otherlight sources and complete a similar flowchartshowing how each source generates light.

Key ConceptsWhat are the six commonsources of light?

How does each type oflight source generatelight?

Vocabulary◆ luminous ◆ incandescent◆ fluorescence◆ phosphor◆ laser◆ coherent light

Incandescent Bulb

Filament radiates light.

Electrons flow through filament.

a. ?

558 Chapter 18

FOCUS

Objectives18.5.1 Explain how light is produced

by common sources of light.18.5.2 Describe the uses of different

light sources.18.5.3 Distinguish lasers from other

light sources.

Build VocabularyWord-Part Analysis Ask students whatwords they know other than those in thevocabulary list that have the key wordparts lum-, in-, fluor-, and co-. (illuminate,inactive, fluoride, and cooperate) Give adefinition of each word part. (Lum-means “light,” in- means “not,” fluor-means “to flow,” and co- means“together.”) Give additional examplesthat share these word parts. (Lumen,inactive, fluorine, and co-author)

Reading Strategya. Filament gets hot. Sample answer:Fluorescent—electric current heatselectrodes, which emit electrons;electrons strike mercury vapor atoms;mercury atoms emit ultraviolet rays;ultraviolet rays strike phosphor coating;phosphors emit visible light.

INSTRUCT

Incandescent LightUse VisualsFigure 27 Have students carefullyexamine the photograph. Ask, Howmany filaments can you see in thisbulb? (Possible answer: It looks like thereare two filaments on top and one morefilament lower down that is not glowing as brightly.) How could a bulb usemore than one filament? (Severalfilaments could be used to vary thebrightness of the bulb, as in a three-way bulb.)Logical, Visual

L1

2

L2

L2

Reading Focus

1

Section 18.5



Technology• Interactive Textbook, Section 18.5• Presentation Pro CD-ROM, Section 18.4• Go Online, Science News, Light and optics

Section Resources


Fluorescent LightIn a process called fluorescence (floo uh RES uns), a materialabsorbs light at one wavelength and then emits light at a longerwavelength. A phosphor is a solid material that can emit lightby fluorescence. Fluorescent light bulbs emit light bycausing a phosphor to steadily emit photons. A fluorescentbulb, such as the one in Figure 28, is a glass tube that containsmercury vapor. Inside, the glass is coated with phosphors.

When electric current flows through a fluorescent bulb,small pieces of metal called electrodes heat up and emit elec-trons. The electrons hit atoms of the mercury vapor, causingthe mercury atoms to emit ultraviolet rays. The ultravioletrays strike the phosphor coating on the inside of the tube andthe atoms emit visible light.

You may have noticed that office buildings and schoolsuse mostly fluorescent lights. Fluorescent tubes do not get ashot as incandescent bulbs because they emit most of theirenergy as light. This means that they use energy very effi-ciently. One 18-watt fluorescent tube provides the sameamount of light as a 75-watt incandescent bulb, and the fluo-rescent tube lasts ten times longer.

What happens during fluorescence?

Glass tube filled with

mercury vapor

Electrode

Electrical contacts

Comparing Fluorescent and Incandescent Light

Analyze and Conclude1. Comparing and Contrasting How do

the spectra produced by incandescent andfluorescent lights compare?

2. Drawing Conclusions During fluorescence,electrons absorb energy and move to specifichigher energy levels. As they move back to alower energy level, they release energy inthe form of light. How does this fact helpexplain the appearance of the spectrum offluorescent light?

Materialsspectroscope, clear incandescent bulb, fluorescentbulb, colored pencils

Procedure1. Turn on a clear, incandescent bulb.

CAUTION Incandescent bulbs get quite hot afterthey have been on for some time. Observe thespectrum of the light coming from theincandescent bulb through a spectroscope.

2. Use colored pencils to draw this spectrum.Label the source of the spectrum.

3. Repeat Steps 1 and 2 with a fluorescent bulb.

Figure 28 The electrodes in afluorescent bulb emit electronsthat cause the mercury atoms toemit ultraviolet rays. These rayscause the phosphor coating toemit light.

Fluorescent Light FYIIn fluorescent light, the phosphors areusually zinc sulfide with traces of copper.

Comparing Fluorescent and Incandescent LightObjectiveAfter completing this activity, studentswill be able to • explain the difference in spectra of

incandescent and fluorescent lights.

Students may think that all white light issimilar and consists of a continuousdistribution of frequencies that includesthe entire visible spectrum. Use theobservations in this activity to help students overcome this misconception.

Skills Focus Observing, Inferring

Prep Time 15 minutes


Safety Caution students that theyshould never directly observe thespectrum of the sun or any other intense light source because it can cause immediate, permanent eyedamage. Caution students to avoidtouching bulbs, which may become hot.

Expected Outcome Students willobserve that incandescent bulbsproduce continuous spectra, while thespectra of fluorescent light sourcesinclude narrow lines of various colors.

Analyze and Conclude1. Incandescent light has a continuousspectrum. The spectrum of fluorescentlight consists of a number of colored lines.

2. The lines are produced by photonswith specific energies emitted byelectrons returning from higher energylevels to the ground state. Visual, Group

L2


Customize for Inclusion Students

Behaviorally DisorderedDivide the class into six groups. Each groupwill be assigned a light source. Explain that thegoal of each group is to find out details abouteach kind of light source. Foster self-esteem byencouraging groups to give individualsownership of particular research topics such as

historical details or specialized uses. Givegroups choices for presenting their results to minimize pressure related to this project.Options include written reports, posters,multimedia presentations, a Web site, or amock science museum exhibit.

Answer to . . .

Figure 27 Hypothesis: A 100-wattbulb is generally brighter because itemits more energy per second.

A material converts UVrays into visible light.


Laser LightA laser is a device that generates a beam of coherent light. The wordlaser stands for light amplification by stimulated emission of radiation.

Laser light is emitted when excited atoms of a solid, liquid, orgas emit photons. Light in which waves have the same wavelength,and the crests and troughs are lined up, is coherent light. A beam ofcoherent light doesn’t spread out significantly from its source, so thelight has a relatively constant intensity. The energy it carries may befocused on a small area.

Lasers can cut through metals and make computer chips. Surgeonsuse lasers to cut or repair damaged tissue. Lasers carry informationthrough optical fibers. Laser light is used to measure distances precisely.

Gas LaserLaser light is produced by exciting theatoms of a solid, liquid, or gas so thatthey emit photons. Some of thesephotons collide with other excited atomsand stimulate the emission of morephotons. Eventually some photons arereleased as an intense beam of light. Thegas laser shown here uses a mixture ofhelium and neon gases. InterpretingPhotographs How does a laser producecoherent light?

Laser beam scansthe label.

Using lasers Hand-held devicesincorporating lasers areused in stores forreading bar codes. Inthe home, they are atthe heart of manydevices such as DVDand CD players.

Fully reflective mirror Photonsbounce between this and a second,

semi-reflective mirror.

Gas mixture Helium and neongases are held in a sealed glass

tube. Additional tubes, containingelectrodes, are attached at the sides.

560 Chapter 18

560 Chapter 18

Laser LightBuild Reading LiteracySQ3R Refer to page 530D in thischapter, which provides guidelines forSQ3R (Study, Question, Read, Recite,Review).

Teach this independent-study skill as awhole-class exercise. Direct students tosurvey the section and write headingssuch as Laser Light. As they survey, askstudents to write one question for eachheading, such as “How does a laser emitlight?” Then, have students write answersto the questions as they read the section.After students finish reading, demonstratehow to recite the questions and answers,explaining that vocalizing in your ownwords helps you retain what you learned.Finally, have students review their notesthe next day.Auditory, Group

Gas LaserMaterial excited in a laser is called lasingmaterial. Lasing material can be a gas(CO2), a gas mixture (helium-neon), a liquid (liquid crystals or organic dyes in a liquid solution or suspension), or asolid crystal (ruby). The lasing materialdetermines the frequency of the lightproduced. Lasers do not have to pro-duce visible light—there are infrared,ultraviolet, and X-ray lasers.

Interpreting Photographs Thewaves in the laser beam all have thesame wavelength and direction of travel, and their peaks coincide. Visual

For EnrichmentInterested students can make amultimedia presentation for the classexplaining an application such as CD players, fiber optic networks, andlaser eye surgery. Verbal, Portfolio

L3

L2

L2


Diverging Laser Beams Lasers do notnecessarily produce parallel rays of light. Atypical pocket diode laser produces red lightthat would diverge like a point source. A lensinside the laser collimates the beam (makesthe rays nearly parallel).

The Lunar Laser Ranging Experiment beganin 1969 when Apollo 11 astronauts leftreflectors on the moon. Scientists send a laserbeam from Earth and measure the time ittakes to detect a return signal. The time delay

gives the distance to the moon to an accuracyof about 3 cm. The experiment has been usedto precisely measure the moon’s orbit andmovement of tectonic plates on Earth. To hit the reflectors, a laser beam is fired in thereverse direction through a telescope pointingtoward the moon. The beam starts out withthe same width as the telescope mirror (a fewmeters). When the beam reaches the moon, ithas diverged to a width of more than 6 km.

Facts and Figures

Neon LightA big city at night is likely aglow in neon lights. Neon lights emitlight when electrons move through a gas or a mixture of gases insideglass tubing. Many lights called neon lights contain gases other thanneon. Often, other gases including helium, argon, and krypton are usedin neon lights. Helium gas gives off a pink light. A mixture of argon gasand mercury vapor produces greenish-blue light. Krypton gas pro-duces a pale violet light. Pure neon emits red light when electrons flowthrough the gas. Each kind of gas emits photons of different energies,and therefore different colors. The different photons emitted combineto give each glowing gas a distinctive color. The color of glass used tomake the tube can also affect the color of the light.

For: Articles on light and optics

Visit: PHSchool.com

Web Code: cce-2185

Electrode An electric currentpasses between this electrode

and its twin on the other side ofthe tube, raising gas atoms to anexcited state. This causes theatoms to release photons.

Photon multiplication Thephotons begin to bounce

back and forth off the mirrors ateither end. Some hit otherexcited atoms, stimulating theemission of additional photons.

Semi-reflectivemirror Because it

is only semi-reflective,this mirror reflects mostof the photons but letsa few of them through.

Laser beam A straight,narrow, intense beam of

coherent light emerges at this end.

Coherent lightThe waves in laser light all have the samewavelength and direction of travel, andtheir peaks coincide. Light with theseproperties is called coherent light.Mirror

Neon atom

Helium atom

Tube wall

Coherent waves

Mirror

Laserbeam


Neon LightBuild Science SkillsInferring

Purpose Students observe and compare the spectra of different neon lights.

Materials spectroscope, neon lights

Safety Caution students never to viewthe sun or other intense light sourceswith a spectroscope because irreversibleeye damage could result. Advise studentsto avoid touching neon lights, which canget quite hot.

Advance Prep To construct aninexpensive spectroscope, tape a 1-cmsquare of diffraction grating over a holepunched in an index card. Attach thecard to cover one end of a smallcardboard or plastic tube. Cover theother end of the tube with a cardcontaining a narrow slit. Students willview spectra through the end of thetube with the diffraction grating.

Procedure Have students take homespectroscopes to observe spectra of neonsigns near their homes. The lights ofneon signs have fairly simple line spectra.Ask students to compare these spectra todetermine which signs contain mixturesof gases or are made of colored glassthat filters out certain colors.

Expected Outcome If the spectrum of a neon light appears to be a combin-ation of two simpler spectra thatstudents have observed, it probablycontains a mixture of the gasesresponsible for the simpler spectra. If two spectra appear identical exceptfor a specific region of one spectrumthat is absent in the other, one of thesigns may be made of colored glass thatfilters out a range of light frequencies. Visual, Kinesthetic

L3


Science News provides studentswith current information on lightand optics.


562 Chapter 18


Reviewing Concepts1. Name six common sources of light.

2. Describe how each type of bulb producesvisible light.

3. Why are fluorescent light bulbs often used inoffice buildings and schools?

4. List three uses for lasers.

5. How are tungsten-halogen bulbs differentfrom incandescent bulbs?

Critical Thinking6. Comparing and Contrasting How are

the six main types of lights similar? Howare they different?

7. Applying Concepts Why do some bulbsheat up more than others?

8. Formulating Hypotheses A friend rubs acompact fluorescent bulb on her shirt on a dryday, and the bulb lights up for a moment.Propose a hypothesis to explain why.

Sodium-Vapor LightSodium-vapor lights contain a small amount of solid sodium,as well as a mixture of neon and argon gases. As electriccurrent passes through a sodium-vapor bulb, it ionizes thegas mixture. The mixture warms up and the heat causes thesodium to change from a solid into a gas. The current of elec-trons knocks electrons in sodium to higher energy levels. Whenthe electrons move back to lower energy levels, the sodiumatoms emit light. Sodium-vapor lights are energy efficient andgive off very bright light. Many streets and parking lots are illu-minated with sodium-vapor lights. Figure 29 shows howsodium-vapor light produced with neon and argon can alterthe color of the objects it illuminates.

Tungsten-Halogen LightTungsten-halogen light is produced in much the same way as incan-descent light. But unlike incandescent lights, a tungsten-halogen’sbulb has a small amount of a halogen gas, such as iodine, bromine, orfluorine. Inside a tungsten-halogen bulb, electrons flow througha tungsten filament. The filament gets hot and emits light. The halo-gen gas reduces wear on the filament, so tungsten-halogen bulbs lastlonger than incandescent bulbs. The bulb of a tungsten-halogen light ismade of quartz, because quartz has a high melting point. If glass wereused, it would start to melt when the bulb got hot.

Figure 29 The yellow color ofsodium-vapor light makes objectslook different than they lookin sunlight.

Energy Review the types of energy inSection 15.1: mechanical, chemical, thermal,electrical, electromagnetic, and nuclear.Then pick two light sources and describethe energy changes in each after you turnon the light.

562 Chapter 18

Sodium-Vapor LightBuild Science SkillsInferring Ask, Why do sodium-vaporlights have less glare than incandes-cent lights? (In sodium-vapor lights, lightcomes from vapor filling the entire tube,rather than from a small filament. The lightis emitted from a larger area and is easierto look at directly.) Logical, Visual

Tungsten-HalogenLightIntegrate HealthFree, protective metal mesh covers havebeen offered to owners of tungsten-halogen lights because the bulbs get sohot. Have students find out how thesecovers can prevent fires. Verbal

ASSESSEvaluate UnderstandingInvite students to write quiz questionswith answers. Announce that goodquestions will be used on the next test.

ReteachMake a flowchart showing how eachlight source emits light. Cut each chartinto pieces and shuffle and distributethem to students. Ask students to putthe pieces back in the correct order.

Sample answer: Incandescent—Electrons flow in wire (electrical energy); moving electrons heat the filament (thermal energy); filamentglows (electromagnetic energy).


L1

L2

3

L2

L2


atoms that emit light when they drop backdown to a lower energy level; Tungsten-halogen: filament gets hot and emits light.3. Fluorescent bulbs are very efficient. 4. Cut through metals; make computer chips;cut or repair tissue in surgery; transmit signalsin fiber-optic cables5. Tungsten-halogen bulbs are hotter and usea mixture of inert and halogen gases.6. Incandescent and tungsten-halogen lightsuse a glowing filament. Fluorescent lightscause phosphors to fluoresce. Lasers emit

coherent light. Neon lights and sodium vaporlights emit light when excited atoms returnto a lower energy level. Refer to pp. 558–562for differences.7. Bulbs that heat a gas or a filament will heatup more than other light sources. 8. Hypothesis: The shirt acquires a netcharge. The shirt’s electric field excitesmercury atoms in the bulb.


1. Incandescent, fluorescent, lasers, neonlights, sodium-vapor, tungsten-halogen2. Incandescent: filament gets hot and emitslight; Fluorescent: UV rays cause phosphors toemit visible light; Laser: excited atoms of asolid, liquid, or gas emit photons of coherentlight; Neon: excited gas atoms emit lightwhen they drop back down to a lower energylevel; Sodium-vapor: a gas mixture ionizes,vaporizing sodium and producing excited

Section 18.1 18.1 Electromagnetic...

Documents

Transcript of Section 18.1 18.1 Electromagnetic...