9

Unit 1

The AtmosphericFilter

Introduction

Earth's atmosphere is essential to life. Thisocean of fluids and suspended particlessurrounds Earth and protects it from thehazards of outer space. It insulates theinhabitants of Earth from the extremetemperatures of space and stops all but thelargest meteoroids from reaching thesurface. Furthermore, it filters out mostradiation dangerous to life. Without theatmosphere, life would not be possible onEarth. The atmosphere contains the oxygenwe breathe. It also has enough pressure sothat water remains liquid at moderatetemperatures.

Yet the same atmosphere that makes lifepossible hinders our understanding ofEarth's place in the universe. Virtually ouronly means for investigating distant stars,nebulae, and galaxies is to collect andanalyze the electromagnetic radiation theseobjects emit into space. But most of thisradiation is absorbed or distorted by theatmosphere before it can reach a ground-based telescope. Only visible light, someradio waves, and limited amounts of infraredand ultraviolet light survive the passagefrom space to the ground. That limitedamount of radiation has given astronomersenough information to estimate the generalshape and size of the universe andcategorize its basic components, but there ismuch left to learn. It is essential to study theentire spectrum rather than just limited

regions of it. Relying on the radiation thatreaches Earth's surface is like listening to apiano recital with only a few of the piano'skeys working.

Unit Goals

• To demonstrate how the components ofEarth's atmosphere absorb or distortincoming electromagnetic radiation.

• To illustrate how important observationsabove Earth's atmosphere are toastronomy.

Teaching Strategy

The following activities use demonstrationsto show how the components of Earth'satmosphere filter or distort electromagneticradiation. Since we cannot produce all ofthe different wavelengths of electromagneticradiation in a classroom, the light from aslide or filmstrip projector in a darkenedroom will represent the complete electro-magnetic spectrum. A projection screenrepresents Earth's surface and objectsplaced between the projector and the screenrepresent the effects of Earth's atmosphere.With the exception of a take-home project,all the demonstrations can be conducted ina single class period. Place the projector inthe back of the classroom and aim ittowards the screen at the front. Try to getthe room as dark as possible before doingthe demonstrations.

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Activity Titles

Clear AirActivity Objective: To demonstrate that gas molecules in the atmosphere absorb

some of the visible light that passes through them.Application: Astronomy, Physical Science

Water In The AirActivity Objective: To show that moisture is present in the atmosphere and

demonstrate how it absorbs visible light.Application: Astronomy, Meteorology, Physical Science

Red Sky, Blue SkyActivity Objective: To illustrate how the gases in the atmosphere scatter some

wavelengths of visible light more than others.Application: Astronomy, Meteorology, Physical Science

Ultraviolet AbsorptionActivity Objective: To show that the atmosphere is transparent to low-energy

ultraviolet light but not to high-energy ultraviolet light.Application: Astronomy, Environmental Science, Physical Science

Particle PollutionActivity Objective: To observe the effects suspended particles of pollution have on

the transmission of visible light.Application: Astronomy, Environmental Science, Meteorology

Particulate SamplerActivity Objective: To obtain a quantitative measurement of the particulate pollution

present in the neighborhood of the students and school.Application: Astronomy, Environmental Science, Meteorology

Heat CurrentsActivity Objective: To show how visible light appears to shimmer, when seen through

rising heat currents.Application: Astronomy, Meteorology, Physical Science

Day and NightActivity Objective: To demonstrate the effects of day and night cycles on

astronomical observations.Application: Astronomy

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Clear AirDescription: Sheets of clear glass are held between a projector and ascreen to show that not all light is transmitted by apparently clearmaterials.

Objective: To demonstrate that gas molecules in the atmosphereabsorb some of the visible light that passes through them.

Materials:

Several small sheets of clear glass or PlexiglassEmery paper (fine)–Use on glass.ProjectorScreenDark room

Procedures:1. Before handling the glass sheets, smooth

sharp edges with emery paper. Skip tothe next step if using plexiglass.

2. Hold up one of the sheets so that thestudents can examine it. Ask them todescribe what they see. At some point,one student will say that the glass isclear. Ask the other students if theyagree that the glass is clear.

3. Darken the room and turn on theprojector.

4. Hold the clear glass between the light andscreen. Observe the distinct shadow castby the edges of the glass and the slightdimness of the light that has passedthrough the glass. Place an additionalsheet of glass on top of the first andobserve any difference in the light as itpasses through double-thick glass. Add athird sheet of glass and repeat.

5. While holding the glass in the projectorbeam, observe if there are any reflectionsof the light around the room.

6. With the room lights back on, observe theedge of the glass. Does light passthrough the edge? What color is theedge? What causes this color?

Discussion:This demonstration provides an analogy forthe light-filtering effects of the atmosphere.The shadow cast by the glass shows thatalthough the glass appears to be clear, itprevents some of the light from reaching thescreen. The glass reflects some light off itsfront surface and absorbs some of the lightthat attempts to pass through it. The effectis similar to what happens with Earth'satmosphere. Part of the visible radiationattempting to reach Earth is reflected by theatmosphere, particularly by clouds, and partis absorbed and scattered by the gases inthe atmosphere.

For Further Research:• Why do the edges of a sheet of glass

usually appear green?• Compare this demonstration with the

following activities: Red Sky, Blue Sky?(page 14) and Resonance Rings (page44).

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Water In The Air

Materials:Shallow dish or an aluminum pie plateEmpty coffee canIce waterWater spray bottleCloud cutout (white cardboard or foamcore)ProjectorProjection screenDark room

Description: The presence of water in the atmosphere isdemonstrated and its light-filtering effects are shown.

Objective: To show that moisture is present in the atmosphere anddemonstrate how it absorbs visible light.

Procedure:Part 1. Fill the coffee can with ice water and

place it in the shallow dish or pie plate.Observe the outside of the can everyminute or two. Water droplets from theair will begin to condense on the outsideof the can.

Part 2. Darken the room and turn on theprojector. Place some clean water in thespray bottle. Adjust the spray to a finemist. Hold the bottle between the

projector and screen and spray.Observe the shadows on the screen castby the fine water droplets.

Part 3. Simulate how clouds block visibleradiation by holding up a cutout of a cloudbetween the projector and the screen.

Discussion:The first demonstration shows that water ispresent in the atmosphere. The capacity tohold water is determined by the atmospherictemperature. Warm air can hold more waterthan cold air. Because the can is chilled bythe ice water, the air immediately surround-ing the can cools. Lowering air temperaturereduces its capacity to hold water, and sothe excess water condenses on the outsideof the can. The amount of water in theatmosphere at any one time, expressed as apercentage of complete saturation, is calledthe relative humidity. Humid air filters out

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much of the infrared portion of theelectromagnetic spectrum. Since air'scapacity to hold moisture drops withtemperature, astronomers build infraredtelescopes on high mountain tops where theair is much cooler and therefore drier than atlower elevations. Infrared telescopes arealso carried on airplanes like NASA'sGerard P. Kuiper Airborne Observatory fromwhich observations can be taken at altitudesabove 12,000 meters. Another goodlocation for viewing is Antarctica because ofits dry air. Telescopes in space gain aneven better view of infrared radiation.

The second demonstration illustrates theeffect of small water droplets on lightpassage. The third demonstration showsthat clouds are very effective filters of visiblelight.

For Further Research:• Use a sling psychrometer or other humidity

measuring device to determine the relativehumidity of the atmosphere. Does theabsolute humidity in the atmospherechange with the air temperature? Is therea difference in the clarity of the atmo-sphere between warm and cold nights?

• Design an experiment to compare thewater capacity of warm and cold air.

• Obtain black and white pictures of Earthfrom space and estimate the total cloudcoverage visible.

• Is it better to locate an observatory on ahigh or low point above sea level? Why?

• Take your class to a science museum thathas exhibits on atmospheric phenomena.

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Red Sky, Blue Sky

Materials:AquariumStirrerFlashlightOpaque card with holeWaterMilkEye dropperDark room

Description: Milky water is used to simulate a sunset and the bluesky.

Objective: To illustrate how the gases in the atmosphere scattersome wavelengths of visible light more than others.

Procedure:1. Fill the aquarium with water and set up

the demonstration as shown in theillustration.

2. Add a few drops of milk to the water andstir the water to mix the two liquids. Youmay have to add more drops to achievethe desired color change effect. Refer tothe discussion for more information.

3. Darken the room and turn on theflashlight.

4. Observe the color of the light coming fromthe flashlight. Next, observe thecolor of the light as it comesdirectly through the aquarium.Observe the color of the liquid fromthe side of the aquarium.

Discussion:One of the standard "why" questionschildren ask is, "Why is the sky blue?"Sunlight has all of the rainbow colors:red, orange, yellow, green, blue, andviolet. Earth's atmosphere containsmolecules of gas that scatter the blue

colors out of the direct path of sunlight andleave the other colors to travel straightthrough. This makes the Sun look yellow-white and the rest of the sky blue. Thiseffect is accentuated when the Sun is low inthe sky. At sunrise and sunset, sunlight hasto penetrate a much greater thickness ofatmosphere than it does when it isoverhead. The molecules and dust particlesscatter almost all of the light at sunrise andsunset—blue, green, yellow, and orange—with only the red light coming directlythrough to your eyes; so, the Sun looks red.Caution: Never stare directly at the Sun.

In this demonstration, the suspendedparticles of milk scatter the light like themolecules in Earth's atmosphere. Whenthe flashlight beam is viewed directlythrough the water, the blue wavelengths oflight are scattered away from the beam oflight, leaving it yellowish. Increasing theamount of milk simulates smog and the Sunwill look red. Viewing the water from the

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• Draw a diagram on a chalkboard oroverhead transparency like the one shownbelow in which you are looking down atEarth from a position far above the NorthPole. Measure the difference inatmospheric thickness the Sun's rays mustpenetrate to reach each location onEarth's surface in the diagram below.Which ray has the greatest distance totravel through the atmosphere to reachEarth's surface?

• Pretend you are standing at each locationlooking toward the Sun. What colorshould the Sun be?

• What is the approximate local time foreach location?

side reveals a very subtle grey-blue hue.Note : Because of individual color sensitivity,some people may not be able to see thebluish hue.

For Further Research:

Sun

light

A

B

CD

E

East West

Ear

th's Rotation

North PoleSunrise

Noon

Midnight

Sunset

Rotation

EARTH

Note: The thickness of Earth'satmosphere is exaggerated for graphicpurposes.

Atom

Sunlight

Scattered Light (re-emitted from atom)

When energized bysunlight, oxygenand nitrogen atomsin the atmospherere-emit (scatter)light in all direc-tions, causing theentire atmosphereabove us to belighted by sunlight.Violet light is

scattered the most and red light the least(1/10th as much). Because our eyes are notvery sensitive to violet light, the sky appearsblue.

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Ultraviolet Absorption

Materials:Ultraviolet light ("black light") - broad

spectrum (may be available from ahigh school science laboratory or arock and mineral collector)

Fluorescent mineral (See note in thediscussion section.)

Window glassDark room

Description: A piece of glass or clear plastic blocks shortwaveultraviolet light.

Objective: To show that the atmosphere is transparent to low-energyultraviolet light but not to high-energy ultraviolet light.

Procedure:1. Darken the room and turn on the

ultraviolet (UV) light. (See Caution at theend of the discussion section.) Direct theUV light's beam onto the fluorescentminerals. Observe the color of theemitted light.

2. Place the glass between the light and themineral and again observe the emittedlight.

Discussion:Certain minerals and a variety of othersubstances fluoresce or emit visible lightwhen illuminated by ultraviolet light.Fluorescence is a process that exchangesultraviolet-light energy for visible-lightenergy. Photons of ultraviolet light arecaptured by electrons orbiting the nuclei ofatoms within those materials. Theelectrons, gaining energy, are boosted toexcited energy states. The electronseventually release this captured energy asvisible light as they return to lower energystates.

Low-energy ultraviolet light—sometimescalled long-wave UV—penetrates to Earth'ssurface. This low-energy ultraviolet lightcauses Day-Glo paints to give offspectacular colors and white clothing toglow brightly when washed in detergentsthat contain fluorescent dyes (advertised asmaking clothes "whiter than white"). Most ofthe high-energy ultraviolet light—sometimescalled short-wave UV—is blocked by theozone layer in Earth's upper atmosphere.Higher-energy ultraviolet light causes skin

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tanning. Extended exposure can lead toeye damage and skin cancer in light-skinnedpeople. Skin cancer is rare in dark-skinnedpeople.

Although transparent to lower-energyultraviolet light, glass blocks higher energyultraviolet light. Lotions that are advertisedas Sun blockers also block higher energyultraviolet light. When the glass is insertedbetween the lamp and the fluorescingmaterial in this demonstration, thefluorescence diminishes or stops. Somematerials fluoresce with lower energyultraviolet waves as well as the higherenergy waves, and any continuedfluorescence is the result of the lowerenergy waves.

Ultraviolet light tells astronomers severalthings. For example, the local neighborhoodof our Sun—within 50 light years—containsmany thousands of low-mass stars that glowin the ultraviolet. When low-mass stars useup all their fuel, they begin to cool. Overbillions of years, the internal heat left overfrom stellar fusion reactions radiates intospace. This leftover heat contains a greatdeal of energy. These stars, called whitedwarfs, radiate mostly ultraviolet light. Untilastronomers could make observations withultraviolet telescopes in space, they hadvery little information about this phase of astar's evolution.

Caution: Do not look into the light emittedby the broad spectrum ultraviolet lamp.

Avoid directing the light to reflectivesurfaces. Everyone should wear eyeprotection such as laboratory safety glassesor ordinary eye glasses.

Where to Obtain Ultraviolet Lights andMinerals:Many science-supply catalogs sell ultravioletlights and fluorescent minerals. If youpurchase a light, be sure to obtain a broadspectrum light because it will emit both longand short wave ultraviolet light. Orderminerals, such as calcite, fluorite, andfranklinite, that fluoresce at shortwavelengths, long wavelengths, and bothlong and short wavelengths. If you do notwish to purchase a lamp and minerals,check with other schools to see if they haveequipment you can borrow. Also check withlocal rock and mineral clubs. Manycollectors have lights and fluorescentminerals and may be willing to come to yourschool to give a demonstration. If ultravioletminerals are not available, experiment withultraviolet-sensitive paints or paper.

For Further Research:• Check recent magazine articles about

problems with Earth's ozone layer andultraviolet radiation at Earth's surface.Learn what can be done to help protectEarth's ozone layer.

• Take a field trip to a science museum thathas displays of fluorescent minerals orarrange for a rock and mineral collector tobring a fluorescent mineral display to yourschool.

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Particulate Pollution

Materials:Stick or book matchesTwo dusty chalk erasersFlashlightProjectorProjection screenDark room

Description: Particulate matter from both natural and human madesources obscures the sky.

Objective: To observe the effects of suspended particles of pollutionon the transmission of visible light.

Procedure:1. Darken the room and turn on the projector.2. While standing in the projector beam,

strike a match. Observe the shadows onthe screen that are created by the smokereleased by the combustion process.

3. Smack two dusty chalk erasers togetherand observe the shadows caused by thechalk dust.

4. Turn off the projector and turn on theflashlight. Stand against a darkbackground and stir up more chalk dust byslapping erasers together. Shine theflashlight's beam on the dust before itsettles. Observe the brightness of thedust particles that are in the beam.

Discussion:Dust and dirt in the atmosphere darken thesky during the daytime. These particulatescome from both human and natural activities.Exhaust from internal combustion enginesmakes carbon monoxide gas, nitrogen oxidegas, and carbon (soot) particles. Industrialand home heating and forest fires, such asslash and burn agriculture, also contribute tothe dust and dirt in the sky. Naturalprocesses, like lightning-caused forest fires

and volcanic eruptions, add large amounts ofdust and ash to the air. All these extraparticles in the air not only filter out sunlight,causing redder sunsets and sunrises, butalso increase "light pollution." Street,residential, and industrial night lighting reflectoff particles and water vapor in theatmosphere to prevent the night sky frombeing really dark. However, many urbanareas have begun to choose street lightingthat interferes less with astronomicalobservations. Because of light pollution,observatories are usually located as far fromurban areas as possible. Survey teamsspend several months on candidatemountain tops observing weather patterns.They study particulates, wind, humidity, andtemperature extensively before committing tothe construction of an observatory.

For Further Research:• Check reference books to learn about the

effects of volcanic eruptions on the clarityof Earth's atmosphere.

• How could street light fixtures be modifiedto reduce light pollution?

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Description: A simple adhesive sampler is taken home by eachstudent to estimate the extent of particulate air pollution present intheir neighborhoods.

Objective: To obtain a quantitative measurement of the particulatepollution present in the neighborhood of the students and school.

Particulate Sampler

the squares by tossing the dice. If thenumbers come up two and five, forexample the square is found in thesecond column, fifth row. Divide the totalnumber of particles counted by 10 to getan average number per square. Eachsquare is two centimeters on a side.

5. Compare the average particle counts tothe locations where the collectors wereplaced (proximity to farms, factories,freeways, etc.). Add the average particlecount for all samplers together and divideby the total number of collectors to obtaina regional average for the two-centimetersquare. Using this average, calculate thetotal number of particles for one squarekilometer of area centering on the school.What would the count be for ten squarekilometers?

* Depending upon local conditions, a longer samplingperiod may be required.

Discussion:Even on a clear night, many small particlesare present in the atmosphere. Dust par-ticles are lofted into the air by wind andother particles are produced as combustionproducts from cars, fire places, industry,

Procedure:1. Tape the graph paper to the center of the

cardboard. Tape the contact paper ontop of the graph paper with sticky side up.Keep the protective backing on thecontact paper.

2. Place the pollution sampler outside on aflat surface, preferably a meter or twoabove the ground. You may have toanchor the sampler if the air is windy.Remove the protective backing. Makesure the contact paper is firmly tapeddown on the cardboard.

3. After exposing the sampler to the outsidefor twenty-four hours, place the graphpaper over the collecting surface, gridside down, and return the sampler toschool.*

4. Remove sampler from the cardboard andobserve the particles from the back sideof the clear Contact Paper. Using themagnifier, count the number of particlesfound in each of ten randomly selectedsquares on the graph paper grid. Select

Materials:Clear contact paper (14 centimeters

square)Graph paper (reproduce copies of

the graph paper provided with theactivity)

Cardboard or 1/4 inch plywood (40centimeters square)

Cellophane tapeMagnifying glassDice

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volcanic eruptions, and a variety of othersources including meteorites and comets.Every day, about 100 tons of meteorite andcomet dust falls to Earth from space. Over-all, dust particles scatter some of the lightthat comes through the atmosphere fromspace. Note: Due to wide variations in airquality, it is recommended that this experi-ment be pre-tested in the school neighbor-hood to learn how long to expose the sam-pler. The time period may have to beextended to several days to showmeasureable results. Although the collectorwill probably collect particles ofextraterrestial origin, telling which particles

Particulate Sampler Grid

Each square is two centimeters on a side.

1 2 3 4 5 6

1

2

3

4

5

6

are extraterrestial will require analyticaltechniques beyond the scope of this guide.

For Further Research:• Contact your local air pollution authority or

the U.S. Environmental Protection Agencyfor additional information about the airquality in your community.

Completed particulate sampler.The grid has been placed over thesticky side of the contact paper.

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Heat CurrentsDescription: Twinkling starlight is simulated by placing a heat sourcein the beam of a slide projector.

Objective: To show how visible light appears to shimmer, when seenthrough rising heat currents.

Procedure: Making a Star Slide1. Obtain a 35 mm slide mount from a

camera store.2. Cut a small square of aluminum foil to fit

the slide frame.3. Using the pin, make about 20 or 30 pin

prick holes in the foil. The slide is readyto be used. Note: An overhead projectorcan also be used. Make the aluminum foilslide large enough to cover the projectorstage.

Procedure: Heat Currents1. Darken the room and turn on the projector

with the star slide. Observe the appear-ance of the stars.

2. Light the food warmer fuel can and place itnear the projector lens between theprojector and the screen.

Caution: The alcohol fuel in the warmer canis ideal for this activity because itproduces heat with little light. Be carefulwhen handling the can in the darkness.

Materials:Food warmer fuel (e.g. Sterno) or

hotplateMatches (if using fuel)Slide projector"Star" slide (See instructions below.) 35 mm slide frame Aluminum foil Pin ScissorsProjection screenDark room 3. Stand back and observe the optical effects

on the screen.

Discussion:Heat currents in the atmosphere cause thetwinkling of starlight. Light rays from stars arerefracted or bent as they pass through cells(masses) of warm, less dense air into cells ofcooler, more dense air. This causes the pathof the light rays to bend slightly many timeseach second, producing the twinkling effect.Heat currents cause focusing problems forastronomical telescopes used for photo-graphy; the star images dance around on thefilm and create fuzzy disks. The use ofimage-sensing and computer processing cannegate the twinkling effect somewhat, but thebest way to minimize the problem is to placetelescopes at high altitudes, such as onmountain tops, or in orbit above Earth'satmosphere.

For Further Research:• Observe the effects of heat currents rising

from asphalt on hot summer days or fromhot water radiators on winter days.

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Day and NightDescription: A bright light bulb simulates the effect that scatteredsunlight has on astronomical observations in the visible region of theelectromagnetic spectrum.

Objective: To demonstrate the effects of day and night cycles onastronomical observations.

Day/night cycles greatly reduce theobserving time available to astronomersemploying optical telescopes based onEarth. Furthermore, on nights when theMoon is full or near full, the Moon'sscattered light greatly interferes with thequality of pictures terrestrial telescopes cangather. You can simulate the Moon's effecton nighttime observation with a dimmerswitch set to a low setting.

For Further Research:• Are there any regions of the electromag-

netic spectrum that are not affected byday/night cycles?

• Compare the number of stars visible on aclear moonless night with the numbervisible when the Moon is full. One way todo this would be to hold up a hoop, suchas an embroidery hoop, at arms length inthe direction of a familiar constellation.Count the stars visible in this location on amoonless night and on a night when theMoon is full.

Materials:Projector"Star" slide (See Heat Currents

activity.)150 or 200 watt light bulb and

uncovered light fixtureProjection screenDark room

Procedure:1. Place the light bulb and fixture near the

projection screen.2. Turn out the room lights and project the

star slide on to the screen.3. When everyone's eyes adjust to the dark

light, turn on the light bulb.4. Observe what happens to the star images

on the screen.

DiscussionIn this demonstration, the light bulb representsthe Sun. Turning on the bulb causes many ofthe star images on the screen to disappear.Images toward the outside of the screen willprobably still be visible. Note: You can also dothis demonstration by simply turning on theroom lights. Using a bright light bulb, however,more closely simulates what happens in thesky.

This demonstration shows that stars do notgo away during the daylight. Instead, lightfrom the Sun is scattered by the gasmolecules, water, and dust particles in ouratmosphere. This scattered light masks thefar dimmer light of stars more distant thanour Sun.

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Introduction

Contrary to popular belief, outer space is notempty. It is filled with electromagneticradiation that crisscrosses the universe.This radiation comprises the spectrum ofenergy ranging from radio waves on oneend to gamma rays on the other. It is calledthe electromagnetic spectrum because thisradiation is associated with electric andmagnetic fields that transfer energy as theytravel through space. Because humans cansee it, the most familiar part of theelectromagnetic spectrum is visible light—red, orange, yellow, green, blue, and violet.

Like expanding ripples in a pond after apebble has been tossed in, electromagneticradiation travels across space in the form ofwaves. These waves travel at the speed oflight—300,000 kilometers per second.Their wavelengths, the distance from wavecrest to wave crest, vary from thousands ofkilometers across, in the case of the longestradio waves, to smaller than the diameter ofan atom, in the cases of the smallest x-raysand gamma rays.

Electromagnetic radiation has properties ofboth waves and particles. What we detectdepends on the method we use to study it.The beautiful colors that appear in a soapfilm or in the dispersion of light from adiamond are best described as waves. Thelight that strikes a solar cell to produce an

electric current is best described as aparticle. When described as particles,individual packets of electromagnetic energyare called photons. The amount of energy aphoton of light contains depends upon itswavelength. Electromagnetic radiation withlong wavelengths contains little energy.Electromagnetic radiation with shortwavelengths contains a great amount ofenergy.

Scientists name the different regions of theelectromagnetic spectrum according to theirwavelengths. (See figure 1.) Radio waveshave the longest wavelengths, ranging froma few centimeters from crest to crest tothousands of kilometers. Microwaves rangefrom a few centimeters to about 0.1 cm.Infrared radiation falls between 700nanometers and 0.1 cm. (Nano means onebillionth. Thus 700 nanometers is adistance equal to 700 billionths or 7 x 10-7

meter.) Visible light is a very narrow band ofradiation ranging from 400 to 700nanometers. For comparison, the thicknessof a sheet of household plastic wrap couldcontain about 50 visible light waves arrangedend to end. Below visible light is the slightlybroader band of ultraviolet light that liesbetween 10 and 300 nanometers. X-raysfollow ultraviolet light and diminish into thehundred-billionth of a meter range. Gammarays fall in the trillionth of a meter range.

The wavelengths of x-rays and gamma rays

Unit 2

TheElectromagneticSpectrum

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Figure 1. Electromagnetic Spectrum

104 2 -3 -5 -7 -10 -12

1 10 101010 10 10-6

10

Wavelengths in Meters

Radio

Gamma rayMicrowave

Infrared

VisibleUltraviolet

X ray

Ångstroms and Nanometers

Astronomers still use an old unit ofmeasurement for the wavelengths ofelectromagnetic radiation. The unit isthe angstrom, or Å, named after theSwedish astronomer who first namedthese wavelengths. One nanometer isequal to 10 angstroms. Therefore, greenlight has a wavelength of about 5000 Å,500 nanometers, or 5 X10-7 meters.

stars and planets, astronomers collectelectromagnetic radiation from them using avariety of tools. Radio dishes capture radiosignals from space. Big telescopes on Earthgather visible and infrared light. Interplanetaryspacecraft have traveled to all the planets inour solar system except Pluto and havelanded on two. No spacecraft has everbrought back planetary material for study.They send back all their information by radiowaves.

Virtually everything astronomers havelearned about the universe beyond Earthdepends on the information contained in theelectromagnetic radiation that has traveledto Earth. For example, when a starexplodes as in a supernova, it emits energyin all wavelengths of the electromagneticspectrum. The most famous supernova isthe stellar explosion that became visible in1054 and produced the Crab Nebula.Electromagnetic radiation from radio togamma rays has been detected from thisobject, and each section of the spectrumtells a different piece of the story.

For most of history, humans used onlyvisible light to explore the skies. With basictools and the human eye, we developedsophisticated methods of time keeping and

are so tiny that scientists use another unit,the electron volt, to describe them. This isthe energy that an electron gains when itfalls through a potential difference, orvoltage, of one volt. It works out that oneelectron volt has a wavelength of about0.0001 centimeters. X-rays range from 100electron volts (100 eV) to thousands ofelectron volts. Gamma rays range fromthousands of electron volts to billions ofelectron volts.

Using The Electromagnetic Spectrum

All objects in space are very distant anddifficult for humans to visit. Only the Moonhas been visited so far. Instead of visiting

Gamma ray

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Figure 2. Transparency of Earth's Atmosphere

MICROWAVE INFARED VIS

IBLE

UV X-RAYS GAMMA RAYS

RADIO

400

200

100

50

25

12

6

3

SEA LEVEL

ALT

ITU

DE

(K

M)

104 2 -2 -4 -6 -8 -10 -12

1 10 101010 10 10 10

Wavelengths (meters)

radio astronomers unexpectedly found coolhydrogen gas distributed throughout theMilky Way. Hydrogen atoms are thebuilding blocks for all matter. The remnantradiation from the Big Bang, the beginningof the universe, shows up in the microwavespectrum.

Infrared studies (also radio studies) tell usabout molecules in space. For example, aninfrared search reveals huge clouds offormaldehyde in space, each more than amillion times more massive than the Sun.Some ultraviolet light comes from powerfulgalaxies very far away. Astronomers haveyet to understand the highly energetic

calendars. Telescopes were invented in the17th century. Astronomers then mappedthe sky in greater detail––still with visiblelight. They learned about the temperature,constituents, distribution, and the motions ofstars.

In the 20th century, scientists began toexplore the other regions of the spectrum.Each region provided new evidence aboutthe universe. Radio waves tell scientistsabout many things: the distribution of gasesin our Milky Way Galaxy, the power in thegreat jets of material spewing from thecenters of some other galaxies, and detailsabout magnetic fields in space. The first

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engines in the centers of these strangeobjects.

Ultraviolet light studies have mapped the hotgas near our Sun (within about 50 lightyears). The high energy end of thespectrum—x-rays and gamma rays—provide scientists with information aboutprocesses they cannot reproduce here onEarth because they lack the required power.So nuclear physicists use strange stars andgalaxies as a laboratory. These objects arepulsars, neutron stars, black holes, andactive galaxies. Their study helps scientistsbetter understand the behavior of matter atextremely high densities and temperaturesin the presence of intense electric andmagnetic fields.

Each region of the electromagneticspectrum provides a piece of the puzzle.Using more than one region of theelectromagnetic spectrum at a time givesscientists a more complete picture. Forexample, relatively cool objects, such asstar-forming clouds of gas and dust, showup best in the radio and infrared spectralregion. Hotter objects, such as stars, emitmost of their energy at visible and ultravioletwavelengths. The most energetic objects,such as supernova explosions, radiateintensely in the x-ray and gamma rayregions.

There are two main techniques for analyzingstarlight. One is called spectroscopy andthe other photometry. Spectroscopyspreads out the light into a spectrum forstudy. Photometry measures the quantity oflight in specific wavelengths or by combiningall wavelengths. Astronomers use manyfilters in their work. Filters help astronomersanalyze particular components of thespectrum. For example, a red filter blocksout all visible light wavelengths except thosethat fall around 600 nanometers.

Unfortunately for astronomical research,Earth's atmosphere acts as a filter to blockmost wavelengths in the electromagneticspectrum. (See Unit 1.) Only small portionsof the spectrum actually reach the surface.(See figure 2.) More pieces of the puzzleare gathered by putting observatories athigh altitudes (on mountain tops) where theair is thin and dry, and by flying instrumentson planes and balloons. By far the bestviewing location is outer space.

Unit Goals

• To investigate the visible light spectrumand the near infrared and ultravioletspectral regions.

• To demonstrate the relationship betweenenergy and wavelength in theelectromagnetic spectrum.

Teaching Strategy

Because of the complex apparatus requiredto study some of the wavelengths of theelectromagnetic spectrum, the visible lightspectrum will be studied in the activities thatfollow. Several different methods fordisplaying the visible spectrum will bepresented. Some of the demonstrations willinvolve sunlight, but a flood or spotlight maybe substituted. For best results, some ofthese activities should be conducted in aroom where there is good control of light.

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Activity Titles

Water PrismActivity Objective: To display the colors of the visible spectrum contained in sunlight.Application: Art, Astronomy, Physical Science

Projecting SpectraActivity Objective: To study the range of color hues in the visible spectrum.Application: Art, Astronomy, Physical Science

Simple SpectroscopeActivity Objective: To construct a simple spectroscope with a diffraction grating.Application: Astronomy, Physical Science

Analytical SpectroscopeActivity Objective: To construct an analytical spectroscope to analyze the spectrum produced when various substances are heated or excited with electricity.Application: Astronomy, Chemistry, Physical Science

Red Shift, Blue ShiftActivity Objective: To demonstrate how stellar spectra can be used to measure a

star's motion relative to Earth along the line of sight.Application: Astronomy, Chemistry, Mathematics, Physical Science

Wavelength and EnergyActivity Objective: To demonstrate the relationship between wavelength, frequency, and energy in the electromagnetic spectrum.Application: Astronomy, Mathematics, Physical Science

Resonance RingsActivity Objective: To show how atoms and molecules in Earth's atmosphere absorb energy through resonance.Application: Astronomy, Environmental Science, Meteorology, Physical Science

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Water PrismDescription: Five glass panes are cemented together and filledwith water to form a water prism that disperses sunlight into itsrainbow of colors.

Objective: To display the colors of the visible spectrum containedin sunlight.

Materials:Glass panes (five) 15x25 cm in sizeSilicone cement (clear)Emery paper (fine)Cellophane tapeWater

Procedure:1. Obtain the glass panes at a hardware or

window store. Have them cut to size.2. Use the emery paper to smooth out the

glass edges to avoid cutting fingersduring handling. (This can be done atthe hardware store.)

3. Temporarily assemble the glass panes asshown in the diagram with cellophanetape. Glue the glass panes together bysmearing each inside joint with siliconecement. This is how aquariums aremade.

4. After the cement dries, remove the tapeand fill the water prism with water.Check for leaks. If leaks are present,empty the water and dry the inside.Cement the leaks and allow the cementto dry.

5. Set the water prism in a window with asunlit exposure. Cover the prism with thefifth pane to help keep the water clean.Sunlight will enter the prism and bedispersed into its rainbow colors. Ob-serve the colors that appear on the flooror wall.

Sunlight

Violet

RedSunlight

Discussion:The water in the water prism bends sunlightthe same way glass does in a glass prism.Sunlight, entering the prism, is bent(refracted). Visible light rays (like sunlight)bend according to their wavelengths.Shorter wavelengths are bent more thanlonger wavelengths. This variable bendingof the light causes its dispersion into thecolors of red, orange, yellow, green, blue,and violet.

The water prism is a good starting point forcapturing student interest in the electromag-netic spectrum. Not only will it disperse abroad swath of color across a classroomwhen sunlight enters it at the proper angle, itis also interesting to look through. Thewater prism disperses narrow bands of coloralong the edges of dark objects.

29

Mirror reflects sunlightinto water prism

Violet

Red

Notes:• Because the altitude of the Sun over the

horizon changes daily, the water prism willwork better at some times of the year thanothers. Sunlight will enter the prism di-rectly in the winter when the Sun angle islow in the sky. In the summer, the Sunangle is very high and its light may notenter the prism at the proper angle fordispersion to take place. This problemcan be corrected by using a mirror toreflect the light at the proper angle.

• Commercial versions of water prisms areavailable from science-supply catalogs.

For Further Research:• What scientist first used the light dispers-

ing properties of the prism to analyzevisible light and color?

• Compare the light dispersion of a glass orplastic prism to the water prism.

• A simpler water prism can be made with ashallow pan of water and a pocket mirror.Refer to the diagram below for set upguidance.

• Learn about rainbows. How is sunlightdispersed by water droplets? What otherhuman-made and natural things dispersesunlight?

Sunlight

Violet

Red

Sunlight

Sunlight

Violet

Red

Water pan and mirror prism Rainbow

Magnified waterdroplet

30

Materials:Method 1

Slide projectorOpaque screen (See instructions.)Glass prismSeveral booksProjection screen

Method 2Overhead projectorHolographic diffraction grating (See next page for sources.)Opaque screen (See instructions.)Several booksTape

Projection screen

Projecting SpectraDescription: Two methods for projecting the visible spectrum areexplained.

Objective: To study the range of colors in the visible spectrum.

Procedure: Method 11. Make an opaque screen approximately

25 cm square from a piece of cardboard,poster board, or wood. Cut a 5 cm-diameter hole out of the middle. Tapetwo pieces of opaque paper or aluminumfoil over the hole so that there is a verticalgap between them that is no wider than 1mm. Stand the screen upright betweentwo books.

2. Arrange the slide projector, opaquescreen, prism, and projection screen asshown in the diagram. Darken the room.Aim the projector's beam at the slot in theopaque screen and adjust the projectorso that the light does not extend aroundthe edges of the opaque screen.

3. Slowly rotate the prism until the narrowslot of light disperses the visible

spectrum. Depending upon the exactalignment, the spectrum may fall on a wallrather than on the screen. Adjust thesetup so that the spectrum is displayedon the projection screen.

Procedure: Method 2.1. For this method, you must obtain a piece

of holographic diffraction grating — agrating produced by accurate holographictechniques. See page 31 for the sourceof the grating. Note: Method 2 will notwork well with a standard transmissiongrating.

2. Make an opaque screen from two piecesof dark paper or other opaque material.Place the pieces on the overheadprojector stage so that there is a narrowslot no wider than 2 mm where light cancome through.

Aluminum foiltaped over holeto form narrowslot

31

3. Darken the room and turn on theprojector. Using tape, hang a piece ofholographic diffraction grating over theupper lens of the projector. Beforetaping, rotate the grating until the bestspectra is displayed. Two brilliant visiblespectra appear on opposite sides of a linerunning from the projector perpendicularto the screen. The size of the display canbe changed by moving the projectorcloser or farther away from the screen.Refer to the Analytical Spectroscopeactivity for more information on how thediffraction grating works.

Discussion:Visible light, passing through a prism at asuitable angle, is dispersed into itscomponent colors. This happens becauseof refraction. When visible light waves crossan interface between two media of differentdensities (such as from air into glass) at anangle other than 90 degrees, the light wavesare bent (refracted). Different wavelengths

Off On

Grating

2 mm slot

Arbor Scientific Flinn ScientificP.O. Box 2750 P.O. Box 219Ann Arbor, MI 48106-2750 131 Flinn StreetPhone: 1-800-367-6695 Batavia, IL 60510

Phone: 1-800-452-1261Learning Technologies, Inc.59 Walden StreetCambridge, MA 02140Phone: 1-800-537-8703.

of visible light are bent different amountsand this causes them to be dispersed into acontinuum of colors. (See diagram.)

Diffraction gratings also disperse light.There are two main kinds of gratings. Onetransmits light directly. The other is amirror-like reflection grating. In either case,diffraction gratings have thousands of tinylines cut into their surfaces. In both kinds ofgratings, the visible colors are created byconstructive and destructive interference.Additional information on how diffractiongratings work is found in the AnalyticalSpectroscope activity and in many physicsand physical science textbooks.

For Further Research:• Use crayons or colored pencils to sketch

the spectra displayed by either projectionmethod. What colors are visible?

• Who discovered the visible spectrum?How many colors did the scientist see?

• A compact audio disk acts like a reflectiondiffraction grating. Darken the room andshine a strong beam of white light from aflashlight on the disk. The beam will bedispersed by the grating and be visible ona wall.

Sources:Holographic diffraction gratings are availablefrom:

Vis

ible

S

pect

rum

White Light

32

Simple SpectroscopeDescription: A basic hand-held spectroscope is made from adiffraction grating and a paper tube.

Objective: To construct a simple spectroscope with a diffractiongrating.

Materials:Diffraction grating 2 cm square (See note.)*Paper tube (tube from toilet paper roll)*Poster board square (5x10cm)*Masking tapeScissorsRazor blade knife2 single edge razor bladesSpectrum tubes and power supply (See note.)Pencil

* per spectroscope

second circle. Use the razor blade knifeto cut across the ends of the cuts to forma narrow slot across the middle of thecircle.

5. Place the circle with the slot against theother end of the tube. While holding it inplace, observe a light source such as afluorescent tube. Be sure to look throughthe grating end of the spectroscope. Thespectrum will appear off to the side fromthe slot. Rotate the circle with the slotuntil the spectrum is as wide as possible.Tape the circle to the end of the tube inthis position. The spectroscope iscomplete.

6. Examine various light sources with thespectroscope. If possible examinenighttime street lighting. Use particular

Procedure:1. Using the pencil, trace around the end of

the paper tube on the poster board.Make two circles and cut them out. Thecircles should be just larger than thetube's opening.

2. Cut a 2 centimeter square hole in thecenter of one circle. Tape the diffractiongrating square over the hole. If studentsare making their own spectroscopes, itmay be better if an adult cuts the squaresand the slot in step 4 below.

3. Tape the circle with the grating inward toone end of the tube.

4. Make a slot cutter tool by taping twosingle edge razor blades together with apiece of poster board between. Use thetool to make parallel cuts about 2centimeters long across the middle of the

33

caution when examining sunlight; do notlook directly into the Sun.

Discussion:Refer to the discussion on AnalyticalSpectroscopes for information on howdiffraction gratings produce spectra.

Glass prisms are heavy. The moreseparation one wants for the wavelengths,the thicker the glass needs to be. Gratingspectroscopes can do the same job but aremuch lighter. A diffraction grating canspread out the spectrum more than a prismcan. This ability is called dispersion.Because gratings are smaller and lighter,they are well suited for spacecraft wheresize and weight are importantconsiderations. Most research telescopeshave some kind of grating spectrographattached. Spectrographs arespectroscopes that provide a record,photographic or digital, of the spectrumobserved.

Notes:• Most science supply houses sell diffraction

grating material in sheets or rolls. Onesheet is usually enough for every studentin a class to have a piece of grating tobuild his or her own spectroscope.Holographic diffraction gratings work bestfor this activity. Refer to the note onsources in the previous activity.

• Many light sources can be used for thisactivity, including fluorescent andincandescent lights and spectra tubes withpower supplies. Spectra tubes and thepower supplies to run them are expensive.It may be possible to borrow tubes andsupplies from another school if your schooldoes not have them. The advantage ofspectrum tubes is that they providespectra from different gases such ashydrogen and helium.

For Further Research:• Using colored pencils or crayons, make

sketches of the spectrum emitted bydifferent light sources. Try incandescentand fluorescent lamps, bug lights, streetlights (mercury, low-pressure sodium, andhigh-pressure sodium), neon signs, andcandle flames. How do these spectradiffer?

• How do astronomers measure the spectraof objects in space? What do thosespectra tell us about these objects?

• Relate this activity to the AnalyticalSpectroscope activity that follows (page34).

34

Analytical SpectroscopeDescription: A spectroscope is constructed that permits theanalysis of visible light.

Objective: To construct an analytical spectroscope to analyzethe spectrum produced when various substances are heated orexcited with electricity.

Materials:Heavy poster board (any color, but

the inside surface should be dark)Holographic diffraction grating (See the Projecting Spectra activity for the source.)Aluminum foilPatterns (See pages 38 and 39.)PencilBlack tapeScissorsRazor blade knifeStraight edgeCutting surfaceSpectrum tubes and power supply (See the discussion section for source.)

Procedure:1. Cut out the patterns for the spectroscope

housing. Trace them on to the heavyposter board. The patterns should bearranged like the sample shown on page39.

2. Cut out the housing from theposter board. Lightly scorethe fold lines with the razorblade knife. (If you should cutall the way through, just tapethe pieces together.)

3. Fold the housing to look like apie shaped-box and tape thecorresponding edges together.

4. Using the razor blade knife,straight edge, and cutting

surface, cut out the slit adjustment andmeasurement scale rectangles from thefront end piece. Also cut out the smallsquare for the diffraction grating holder.

5. Cut a piece of diffraction grating largeenough to cover the hole in the eyepiecerectangle. Handle the grating by theedges if possible; skin oils will damage it.Look at a fluorescent light through thegrating. Turn the grating so that therainbow colors you see appear in fatvertical bars to the right and left of thelight. Tape the grating to the back side ofthe eyepiece rectangle in this sameorientation. Refer to the pattern page formore information on the alignment of thegrating.

6. Tape the eyepiece rectangle to thenarrow end of the spectroscope housing.

7. Cut out the black measurement grid fromthe paper. Tape this grid to the inside ofthe narrow rectangle cut out in step 4.Carefully align the grid with the hole sothat when you hold the front end piece tothe light, the grid will be illuminated by the

35

light passing through the paper. Trim anyexcess tape.

8. Tape the front end panel to the wideopening of the spectroscope housing.Make sure the seams are closed so thatno light escapes through them.

9. The final step in making the spectroscopeis to close off the other open rectangleexcept for a very narrow vertical slot. Theslit can be cut from aluminum foil. Itshould be about one millimeter wide. Usea razor knife to make this cut. Anyroughness in the cut will show up as darkstreaks in the spectrum perpendicular tothe slot. Cover the open rectangle on thefront piece with the foil. The slit should bevertical. Temporarily hold the paper inplace with tape.

Calibrating the Spectroscope:1. To obtain accurate wavelength readings

with the spectroscope, it must becalibrated. This is accomplished bylooking at a standard fluorescent light (Donot use a broad-spectrum fluorescentlight.)

2. Lift the tape holding the slit and move theslit to the right or left until the bright greenline in the display is located at 546nanometers. Retape the slit in place.The spectroscope is calibrated.

Discussion:Unlike a prism, which disperses white light intothe rainbow colors through refraction, thediffraction grating used in this spectroscopedisperses white light through a process calledinterference. The grating used in this activityconsists of a transparent piece of plastic withmany thousands of microscopic parallelgrooves. Light passing between thesegrooves is dispersed into its componentwavelengths and appears as parallel bands ofcolor on the retina of the eye of the observer.

Spectroscopes are important tools forastronomy. They enable astronomers toanalyze starlight by providing a measure of

the relative amounts of red and blue light astar gives out. Knowing this, astronomers candetermine the star's temperature. They alsocan deduce its chemical composition, estimateits size, and even measure its motion towardor away from Earth (See the activity Red Shift,Blue Shift.)

Starlight (photons) originates from the interiorof a star. There, pressures are enormous andnuclear fusion is triggered. Intense radiation isproduced as atoms, consisting of a nucleussurrounded by one or more electrons, collidewith each other millions of times each second.The number of collisions depends upon thetemperature of the gas. The higher thetemperature, the greater the rate of collisions.

Because of these collisions, many electronsare boosted to higher energy levels, a processcalled excitation. The electrons spontane-ously drop back to their original energy level.In doing so, they release energy as photons.This is what happens to the filament of anelectric light bulb or to an iron bar when it isheated in a furnace. As the temperature ofthe filament rises, it begins to radiate reddishlight. When the filament becomes muchhotter, it radiates bluish light. Thus, the color itradiates is an indicator of the filament'stemperature. Stars that radiate a greatamount of red light are much cooler than starsthat radiate a great amount of blue light.Stellar spectra therefore serve as starthermometers.

Excitation of electrons can also occur if theyabsorb a photon of the right wavelength. Thisis what happens when certain materials areexposed to ultraviolet light. These materialsthen release new photons at differentwavelengths. This is called fluorescence.

One of the important applications ofspectroscopes is their use for identifyingchemical elements. Each element radiateslight in specific wavelength combinations thatare as distinctive as fingerprints. Knowing the

36

"spectral signatures" of each element enablesastronomers to identify the elements present indistant stars by analyzing their spectra.

There are three kinds of spectra: continuous,absorption, and emission. The continuousspectrum appears as a continuous band ofcolor ranging from red to violet when observedthrough a spectroscope. An absorptionspectrum occurs when the light from a starpasses through a cloud of gas, hydrogen forexample, before reaching the spectroscope.As a result, some wavelengths of light areabsorbed by the hydrogen atoms. Thisselective absorption produces a spectrum thatis a broad band of color interrupted by darklines representing certain wavelengths of lightthat were absorbed by the hydrogen cloud.Such a situation occurs when a star is locatedinside or behind a gas cloud or nebula. Anemission spectrum is observed when energy isabsorbed by the gas atoms in a nebula and isreradiated by those atoms at specificwavelengths. This spectrum consists of brightlines against a black background. The lightfrom fluorescent tubes and neon lights produceemission spectra.

Stellar spectra allow astronomers to determinestar temperature, chemical composition, andmotion along the line of sight. This enablesastronomers to classify stars into spectralcategories and estimate their age, reconstructtheir histories, and postulate their futureevolution. When available, astronomers preferstellar spectra collected by orbiting spacecraftover spectra collected by Earth-basedtelescopes since they are not affected byatmospheric filtering and are therefore moreaccurate. Included in the spectra collected byspacecraft are infrared, ultraviolet, x-ray, andgamma ray bands that simply do not reachground-based spectroscopes.

Notes:• This spectroscope works better with a

holographic diffraction grating than withstandard diffraction gratings. Refer to thesource for holographic gratings listed in theProjecting Spectrums activity.

• This spectroscope can be used to analyzethe wavelengths of light from many lightsources. Compare incandescent light,fluorescent light, and sunlight. If you havespectrum tubes and a power supply(available from science supply houses),examine the wavelengths of light producedby the different gases in the tubes. Manyhigh school physics departments have thisequipment and it may be possible to borrowit if your school does not. Use thespectroscope to examine neon signs andstreet lights. Science supply houses sellspectrum flame kits consisting of varioussalts that are heated in the flame of aBunsen burner. These kits are much lessexpensive than spectrum tubes but are moredifficult to work with because the flames donot last very long.

For Further Research:• Compare the solar spectrum at midday and

at sunset. Are there any differences?Caution: Be careful not to look directly at theSun.

• What do spectra tell us about the nature ofstars and other objects in space?

• Show how temperature and radiation arerelated by connecting a clear light bulb to adimmer switch. Gradually increase thecurrent passing through the filament byturning up the dimmer. Observe the colorand brightness of the filament as thetemperature of the filament climbs withincreasing current.

• Ask students to identify what elements arepresent in the object that produced the"Spectra of Unknown Composition" on page37, and to explain their method. How doesthis activity relate to the way astronomersuse spectra to identify the composition of astar?

37

He I

KI.S.

N II O

II

Si IV Si IV

Ca II O

IIO

IIO

II

C II Mg

II

N II

O IISi II

I

H9 H8 Hε

He I

He I Hβ He IHδ

He I Hγ

He I

495 nm370 nm

Partial Spectra of an O Type Star

Spectra of Various Elements

400 500 600 700450 550 650 750

Calcium

Lithium

Sodium

Hydrogen

Helium

Krypton

Nitrogen

Barium

Oxygen

Argon

Spectra of Unknown Composition

38

Housing top and bottom panels (Cut two.)

39

70

05

00

60

04

00

Cut Out

Slit

adj

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rect

angl

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ut.

Mea

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. C

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. Thi

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nel

(Cut

two.

)Housing top and bottom panels (Cut two.)

Hous

ing si

de pa

nel (

Cut tw

o.)

Housing top and bottom

panels (Cut tw

o.)

Housing side panel (Cut two.)

Score folding lines.

Join housing panel patterns like this. Cut from a single piece of posterboard and fold.

Eyepiece Rectangle

Mount diffraction grating over hole so that the light disperses in fat bars to the right and left.

40

Materials:Plastic whiffle ball (12-15 cm in diameter)Microswitch*Small buzzer*9 volt battery*9 volt battery clip*Cord (3 meters long)Solder and soldering ironSharp knife or hacksaw bladeMasking tape

*See note about electronic parts.

Red Shift, Blue Shift

Description: A whiffle ball containing a battery operated buzzer istwirled in a circle to demonstrate the Doppler effect. This sameeffect causes starlight to shift to the blue or red end of the spec-trum if a star is moving towards or away from us.

Objective: To demonstrate how stellar spectra can be used tomeasure a star's motion relative to Earth along the line of sight.

Procedure:1. Splice and solder the buzzer, battery clip,

and microswitch in a simple series circuit.See the wiring diagram on page 42. Besure to test the circuit before soldering.Many small buzzers require the electriccurrent to flow in one direction and will notwork if the current flows in the otherdirection.

2. Split the whiffle ball in half along the seamwith the knife or saw blade.

3. Remove the nut from the microswitch andinsert the threaded shaft through one ofthe holes as shown in the diagram. If ahole is not present in the location shown,use a drill to make one the correct diam-eter. Place the nut back over thethreaded shaft on the microswitch andtighten.

4. Join the two halves of the ball togetherwith the switch, buzzer, and battery in-

side. Tape the halves securely together.5. Tie one end of the cord to the ball as

shown.6. Station students in a circle about 6 meters

in diameter. Stand in the middle of thestudents, turn on the buzzer, and twirl theball in a circle. Play out two to threemeters of cord.

7. Ask the students to describe what theyhear as the ball moves towards and awayfrom them.

8. Let different students try twirling the ball.Ask them to describe what they hear.

As an alternate suggestion to the wiffleball, cut a cavity inside a foam rubber balland insert the battery and buzzer. Theball can then be tossed from student tostudent while demonstrating the Dopplereffect.

41

DiscussionThis is a demonstration of the phenomenoncalled the Doppler effect. It results from themotion of a source (star) relative to theobserver and causes its spectra to beshifted toward the red (going away) ortoward the blue (coming towards) end of thespectra.

Like light, sound travels in waves and there-fore provides an excellent model of thewave behavior of light. The number ofwaves reaching an observer in one secondis called the frequency. For a given speed,frequency depends upon the length of thewave. Long waves have a lower frequencythan short waves. As long as the distancebetween the source of the waves and theobserver remains constant, the frequencyremains constant. However, if the distancebetween the observer and the source isincreasing, the frequency will decrease. Ifthe distance is decreasing, the frequencywill decrease.

Imagine that you are at a railroad crossingand a train is approaching. The train isblowing its horn. The sound waves comingfrom the horn are squeezed closer together

Higher Frequency(Blue Shift)

Lower Frequency(Red Shift)

than they would be if the train were still,because of the train's movement in yourdirection. This squeezing of the wavesincreases the number of waves (increasesthe frequency) that reach your ear everysecond. But after the train's engine passesthe crossing, the frequency diminishes andthe pitch lowers. In effect, the sound wavesare stretched apart by the train's movementin the opposite direction. As the observer,you perceive these frequency changes aschanges in the pitch of the sound. Thesound's pitch is higher as the train ap-proaches and lower as it travels away. Theillustration below provides a graphical repre-sentation of what happens.

A similar situation takes place with stars. Ifthe distance between a star and Earth isincreasing, the lines in the absorption oremission spectrum will shift slightly to thelower frequency, red end of the spectrum. Ifthe distance is decreasing, the lines will shifttoward the blue end. The amount of thatshift can be measured and used to calculatethe relative velocity using the followingformula.

42

Note: A single line of the emission spectrais used for the calculation.

Vr

c= ∆λ

λ0

Vr - radial velocity of the source with respectto the observer.c - speed of light (3 x 105 km/sec)∆λ - the amount of the shift in nanometersλ0 - unshifted wavelength in nanometers

For example, if a line in a spectrum shouldfall at 600 nanometers but instead lies at600.1, what would the radial velocity be?

Vr

= 0.1nm × 3 × 105 / sec

600nm= 50km / sec

The solution to this equation only tells us thevelocity of the source relative to thespectroscope. Whether the distance isincreasing or decreasing is revealed by thedirection of the shift to the red or blue end ofthe visible spectrum. It does not tell, how-ever, if one or both objects are movingrelative to some external reference point.

For Further Research• Does the person twirling the whiffle ball

hear the Doppler shift? Why or why not?• Can the red/blue shift technique be used

for objects other than stars? Can you tellwhich way an emergency vehicle is travel-ing by the pitch of its siren?

• Transverse velocity is a motion that isperpendicular to radial velocity. Can thismotion be detected by the Doppler effect?

• What has Doppler shift told astronomersabout the size of the universe?

Blue Shifted

400 500 600 700450 550 650 750

Simplified Star Spectrum

Red Shifted

Note about Electronic Parts:The electronic parts for this device are notspecified exactly since there are manycombinations that will work. Go to an elec-tronic parts store and select a buzzer, bat-tery holder, battery, and switch from what isavailable. Remember to purchase partsthat will fit in a whiffle ball. The store clerkshould be able to help you make a workableselection if you need assistance. If possible,test the buzzer before purchasing it todetermine if it is loud enough. Test thebuzzer and battery before solderingconnections. The buzzer may be polarized.Reverse the connections if you do not heara sound the first time.

Switch

Battery

Buzzer

43

Wavelength and EnergyDescription: Shaking a rope permits students to feel therelationship between wavelength, frequency, and energy.

Objective: To demonstrate the relationship between wavefrequency and energy in the electromagnetic spectrum.

Wavelength

Wavelength

Materials:Rope (50 ft. length of cotton clothes line)

Procedure:1. Select two students to hold the rope.

Have each student stand in an aisle or inopposite corners so that the rope isstretched between them.

2. While one end of the rope is held still,have the other student shake theopposite end up and down at a moderatebut steady rate.

3. Ask the remaining students to observethe wave patterns created in the rope.Point out wave crests. Ask the studentsto estimate the wavelength andfrequency of waves reaching the otherstudent. The wavelength is the distancefrom wave crest to wave crest.Frequency is the number of wavesreaching the far end of the rope eachsecond.

4. Tell the student shaking the rope to shakeit faster. Again estimate the wavelengthand frequency.

5. Tell the student shaking the rope to shakethe rope as fast as he or she can. Again,estimate the wavelength and frequency.

6. Stop the demonstration and ask thestudent shaking the rope if it is easier toproduce low frequency (long wavelength)or high frequency (short wavelength)waves.

Discussion:This activity provides a graphicdemonstration of the relationship betweenenergy and wavelength. High-frequencywaves (short wavelength) represent moreenergy than low-frequency (long wave-length) waves.

For Further Research:• As time permits, let the remaining students

try the rope demonstration.• A similar demonstration can be conducted

with a coiled spring (Slinky).• Invite a hospital medical imaging specialist

to talk to the class about the use of high-frequency electromagnetic waves inmedical diagnosis.

• Make an overhead projector transparencyof the spectrum chart on page 24. Ask thestudents to relate energy to the electro-magnetic wavelengths depicted.

44

Description: Different sized rings demonstrate the relationshipbetween the frequency of electromagnetic radiation andatmospheric absorption.

Objective: To show how atoms and molecules in Earth'satmosphere absorb energy through resonance.

Resonance Rings

Materials:Used lightweight file foldersCardboard sheet about 20 by 30 cmMasking tapeScissors

Procedure:1. Cut two strips of paper from used file

folders. Each strip should be 3 cm wide.Make the strips approximately 30 and 35cm long.

2. Curl each strip into a cylinder and tape theends together.

3. Tape the cylinders to the cardboard asshown in the diagram. If the ring has acrease from the file folder, the creaseshould be at the bottom.

4. Holding the cardboard, slowly shake it backand forth and observe what happens whenyou gradually increase the frequency of theshaking.

Discussion:All objects have a natural frequency at whichthey vibrate. When the frequency of theshaking matches the frequency of one of therings in this activity, it begins to vibrate morethan the rest. In other words, some of theenergy in the shaking is absorbed by that ring.This effect is called resonance. Resonancetakes place when energy of the rightfrequency (or multiples of the right frequency)is added to an object causing it to vibrate.When electromagnetic radiation enters Earth'satmosphere, certain wavelengths match thenatural frequencies of atoms and molecules ofvarious atmospheric gases such as nitrogen

and ozone. Whenthis happens, theenergy in thosewavelengths isabsorbed bythose atoms ormolecules,intercepting this energy before it reachesEarth's surface. Wavelengths that do notmatch the natural frequencies of theseatmospheric constituents pass through. (Seefigure 2 on page 25. )

Resonance is important to astronomy foranother reason. All starlight begins in thecenter of the star as a product of nuclearfusion. As the radiation emerges from thephotosphere or surface of the star, somewavelengths of radiation may be missing. Themissing components produce dark lines, calledabsorption lines, in the star's spectra. Thelines are created as the radiation passesthrough the outer gaseous layers of the star.Some of that radiation will be absorbed asvarious gas atoms present there resonate.Absorption lines tell what elements are presentin the outer gaseous layers of the star.

For Further Research:• Investigate the natural frequencies of various

objects such as bells, wine goblets, andtuning forks. If you have an oscilloscope, useit to convert the sounds to wave forms.

• Why has the playing of the song "Louie,Louie" been banned at several collegefootball stadiums? Why do marching soldierscrossing a bridge "break cadence"?

• What gas in Earth's upper atmosphere blocksultraviolet radiation? Why is it important?

45

Unit 3

CollectingElectromagneticRadiation

Introduction

Except for rock samples brought back fromthe Moon by Apollo astronauts, cosmic rayparticles that reach the atmosphere, andmeteorites that fall to Earth, the onlyinformation about objects in space comes toEarth in the form of electromagneticradiation. How astronomers collect thisradiation determines what they learn from it.The most basic collector is the human eye.The retina at the back of the eye is coveredwith tiny antennae, called rods and cones,that resonate with incoming light. Reson-ance with visible electromagnetic radiationstimulates nerve endings, which sendmessages to the brain that are interpretedas visual images. Cones in the retina aresensitive to the colors of the visiblespectrum, while the rods are most sensitiveto black and white.

Until the early 1600s, astronomers had onlytheir eyes and a collection of geometricdevices to observe the universe andmeasure locations of stellar objects. Theyconcentrated on the movements of planetsand transient objects such as comets andmeteors. However, when Galileo Galileiused the newly invented telescope to studythe Moon, planets, and the Sun, our know-ledge of the universe changed dramatically.He was able to observe moons circlingJupiter, craters on the Moon, phases of

Venus, and spots on the Sun. Note: Galileodid his solar observations by projecting lightthrough his telescope on to a whitesurface—a technique that is very effectiveeven today. Never look at the Sundirectly with a telescope!

Galileo's telescope and all opticaltelescopes that have been constructed sinceare collectors of electromagnetic radiation.The objective or front lens of Galileo'stelescope was only a few centimeters indiameter. Light rays falling on that lenswere bent and concentrated into a narrowbeam that emerged through a second lens,entered his eye, and landed on his retina.The lens diameter was much larger than thediameter of the pupil of Galileo's eye, so itcollected much more light than Galileo'sunaided eye could gather. The telescope'slenses magnified the images of distantobjects three times.

Since Galileo's time, many huge telescopeshave been constructed. Most haveemployed big mirrors as the light collector.The bigger the mirror or lens, the more lightcould be gathered and the fainter the sourcethat the astronomer can detect. The famous5-meter-diameter Hale Telescope on Mt.Palomar is able to gather 640,000 times theamount of light a typical eye could receive.The amount of light one telescope receivescompared to the human eye is its light

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gathering power (LGP). Much larger eventhan the Hale Telescope is the KeckTelescope that has an effective diameter of10 meters. Its light gathering power is twoand a half million times the amount a typicaleye can receive. Although NASA's HubbleSpace Telescope, in orbit above Earth'satmosphere, has only a LGP of 144,000, ithas the advantage of an unfiltered view ofthe universe. Furthermore, its sensitivityextends into infrared and ultravioletwavelengths.

Once the telescope collects the photons, thedetection method becomes important.Telescopes are collectors, not detectors.Like all other telescopes, the mirror of theHubble Space Telescope is a photoncollector that gathers the photons to a focusso a detector can pick them up. It hasseveral filters that move in front of thedetector so that images can be made atspecific wavelengths.

In the early days, astronomers recordedwhat they saw through telescopes bydrawing pictures and taking notes. Whenphotography was invented, astronomersreplaced their eyes with photographicplates. A photographic plate is similar to thefilm used in a modern camera except thatthe emulsion was mounted on glass platesinstead of plastic. The glass plate collectedphotons to build images and spectra.Astronomers also employed the photo-multiplier tube, an electronic device forcounting photons.

The second half of this century saw thedevelopment of the Charge Coupled Device(CCD), a computer-run system that collectsphotons on a small computer chip. CCDshave now replaced the photographic platefor most astronomical observations. Ifastronomers require spectra, they insert aspectrograph between the telescope and theCCD. This arrangement provides digitalspectral data.

Driving each of these advances is the needfor greater sensitivity and accuracy of thedata. Photographic plates, still used forwide-field studies, collect up to about 5percent of the photons that fall on them. ACCD collects 85 to 95 percent of thephotons. Because CCDs are small and canonly observe a small part of the sky at atime, they are especially suited for deepspace observations.

Because the entire electromagnetic spec-trum represents a broad range of wave-lengths and energies (See figure 1, page24.), no one detector can record all types ofradiation. Antennas are used to collectradio and microwave energies. To collectvery faint signals, astronomers use largeparabolic radio antennas that reflect incom-ing radiation to a focus much in the sameway reflector telescopes collect and concen-trate light. Radio receivers at the focusconvert the radiation into electric currentsthat can be studied.

Sensitive solid state heat detectors measureinfrared radiation, higher in energy andshorter in wavelength than radio and micro-wave radiation. Mirrors in aircraft, balloons,and orbiting spacecraft can concentrateinfrared radiation onto the detectors thatwork like CCDs in the infrared range. Be-cause infrared radiation is associated withheat, infrared detectors must be kept at verylow temperatures lest the telescope's ownstored heat energy interferes with the radia-tion coming from distant objects.

Grazing-incidence mirrors that consist of amirrored cone collect ultraviolet radiationreflected at a small "grazing" angle to themirror surface and direct it to detectorsplaced at the mirror's apex. Different mirrorcoatings are used to enhance the reflectivityof the mirrors to specific wavelengths.

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resolve into two. Since telescopes, forexample, have the effect of increasing thepower of our vision, they improve ourresolution of distant objects as well. Thedesign and diameter of astronomicalinstruments determines whether theresolution is high or low. For stellar work,high resolution is important so theastronomer can study one star at a time.For galaxy work the individual stars in agalaxy may often not be as important as thewhole ensemble of stars.

Unit Goals

• To demonstrate how electromagneticradiation can be collected and detectedthrough the use of mirrors, lenses, infrareddetectors, and radio antennas.

• To illustrate how the use of large instru-ments for collecting electromagnetic radia-tion increases the quantity and quality ofdata collected.

Teaching Strategy

Because many of the wavelengths in theelectromagnetic spectrum are difficult ordangerous to work with, activities in thissection concentrate on the visible spectrum,the near infrared, and radio wavelengths.Several of the activities involve lenses andmirrors. The Lenses and Mirrors activityprovides many tips for obtaining a variety oflenses and mirrors at little or no cost.

X-ray spacecraft also use grazing-incidencemirrors and solid state detectors whilegamma ray spacecraft use a detector of anentirely different kind. The ComptonGamma-Ray Observatory has eight 1-meter-sized crystals of sodium iodide that detectincoming gamma rays as the observatoryorbits Earth. Sodium iodide is sensitive togamma rays, but not to optical and radiowavelengths. The big crystal is simply adetector of photons—it does not focus them.

Today, astronomers can choose to collectand count photons, focus the photons tobuild up an image, or disperse the photonsinto their various wavelengths. High energyphotons are usually detected with countingtechniques. The other wavelengths aredetected with counting (photometry),focusing methods (imaging), or dispersionmethods (spectroscopy). The particularinstrument or combination of instrumentsastronomers choose depends not only onthe spectral region to be observed, but alsoon the object under observation. Stars arepoint sources in the sky. Galaxies are not.So the astronomer must select a combin-ation that provides good stellar images orgood galaxy images.

Another important property of astronomicalinstruments is resolution. This is the abilityto separate two closely-space objects fromeach other. For example, a pair ofautomobile headlights appear to be onebright light when seen in the distance alonga straight highway. Close up, the headlights

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Activity Titles

Pinhole ViewerActivity Objective: To demonstrate how a pinhole viewer inverts light passing

through it.Application: Physical Science, Photography

Build Your Own TelescopeActivity Objective: To build a simple astronomical telescope from two lenses and

some tubes.Application: Astronomy, Physical Science

Reflecting TelescopesActivity Objective: To illustrate how a reflecting telescope works and how it inverts

an image.Application: Astronomy, Physical Science

Lenses and MirrorsActivity Objective: To show how lenses and mirrors can bend and reflect light waves.Application: Astronomy, Mathematics, Physical Science

Light Gathering PowerActivity Objective: To determine the ability of various lenses and mirrors to gather light.Application: Astronomy, Mathematics

Liquid Crystal IR DetectorActivity Objective: To experiment with one method of detecting infrared radiation.Application: Astronomy, Chemistry, Physical Science

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Pinhole ViewerDescription: A pinhole viewer demonstrates how inverted imagesare produced.

Objective: To demonstrate how a pinhole viewer inverts lightpassing through it.

Materials:Cylindrical cereal boxTracing paperRubber bandCandleSharp knifePinAluminum foilCellophane tapeDouble convex lens (magnifying glass)Dark room

Procedure:1. Using the knife, cut a one centimeter

square hole in the center of the bottom ofthe cereal box.

2. Cover the hole with a small piece ofaluminum foil and tape in place. Poke thecenter of the foil with the pin to make ahole directly over the hole made in step 1.

3. Cover the open end of the box with tracingpaper and fasten with a rubber band.

4. Light the candle and darken the room.5. Aim the pinhole viewer so that the pinhole

is pointed at the candle. Observe theimage on the tracing paper.

6. Compare the image produced by thepinhole viewer with the image seen througha double convex lens.

Discussion:Convex lenses invert images. Light raysconverge behind these lenses and cross atthe focal point. At that point, the image

becomes inverted. (See diagram on nextpage.) With the pinhole viewer, the changetakes place at the pinhole.

Astronomical telescopes produce imageslike those of the pinhole viewer. Becausethe big lens or mirror of astronomicaltelescopes are large, a great amount of lightis focused on the detector, producing amuch brighter image than the pinholecamera. Although additional lenses ormirrors can be used to "right" the image,astronomers prefer not to use them. Eachadded lens or mirror removes a smallamount of light. Astronomers want theirimages as bright as possible and do notmind if the images of stars and planets areinverted. Since most large astronomicaltelescopes use photographic film or CCDsas detectors, righting the image, ifnecessary, is merely a matter of inverting

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the negative before making a print orcommanding the computer to right theimage.

For Further Research:• The pinhole viewer can be converted into a

simple camera by placing a piece ofunexposed black and white print filmwhere the tracing paper screen is located.This must be done in a darkroom. Reducelight leaks by covering the film end of theviewer and the pinhole. Point the cameraat an object to be photographed and

uncover the pinhole for several seconds.Recover the pinhole and develop the filmin a dark room. Take several pictures withdifferent exposure lengths until a suitableexposure is found. Show how thenegative is inverted to correct the imagebefore printing.

Images seen through lenses, such as this double convex lens, are inverted. This is similarto the function of the pinhole in the pinhole viewer.

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Materials:Paper mailing tube (telescoping - 1 inside tube and 1 outside tube)Styrofoam trays (1 large and 1 small)Lenses (1 large and 1 small. See note about lenses.)Metric rulerRazor blade knifeCutting surfaceMarker penRubber cementFine grade sandpaper

Procedure:1. Cut a short segment from the end of the

outside tube. This circle will be used fortracing only. Place the circle from thelarger tube on the large tray. Using amarker pen, trace the inside of the circleon to the bottom of the tray three times.

2. Lay the large (objective) lens in the centerof one of the three large circles. Tracethe lens’ outline on the circle.

3. Cut the circle with the lens tracing fromthe tray using the razor blade knife. Besure to place the styrofoam on a safecutting surface. Cut out the lens tracing,but when doing so, cut inside the line sothat the hole is slightly smaller than thediameter of the lens.

4. Before cutting out the other two largecircles, draw smaller circles inside themapproximately equal to 7/8ths of the

Description: A simple refractor telescope is made from a mailingtube, styrofoam tray, rubber cement, and some lenses.

Objective: To build a simple astronomical telescope from twolenses and some tubes.

Build Your Own Telescope

diameter of the large lens. Cut out bothcircles inside and out.

5. Coat both sides of the inner circle (theone that holds the lens) with rubbercement and let dry. Coat just one sideeach of the other two circles with cementand let dry. For a better bond, coat againwith glue and let dry.

6. Insert the lens into the inner circle andpress the other circles to eitherside. Be careful to align thecircles properly. Becausethe outside circleshave smallerdiametersthan thelens, thelens isfirmly heldin place. You have completed theobjective lens mounting assembly.

7. Repeat steps 1- 6 for the inside tube anduse the smaller lens for tracing.However, because the eyepiece lens isthinner than the objective lens, cut theinner circle from the small tray. The foamof this tray is thinner and better matchesthe thickness of the lens.

8. After both lens mounting assemblies are

Outer Tube

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eyepiece. Passing through the eyepiece,the light is refracted again.

This refraction inverts the image. To havean upright image, an additional correctinglens or prism is placed in the optical path.Astronomers rarely care if images are right-side-up or up-side-down. A star looks thesame regardless of orientation. However,

correcting images requires the use of extraoptics that diminish the amount of lightcollected. Astronomers would rather havebright, clear images than right-side-upimages. Furthermore, images can becorrected by inverting and reversingphotographic negatives or correcting theimage in a computer.

Notes About Lenses and Tubes:Refer to the Lenses and Mirrors activity forinformation on how to obtain suitable lensesfor this activity. PVC plumbing pipes can beused for the telescoping tubes. Purchasetube cutoffs of different diameters at ahardware store.

For Further Research:• If the focal lengths of the two lenses used

for the telescope are known, calculate thepower of the telescope. Magnificatione