SOUND WAVES AND ELECTROMAGNETIC RADIATIONChapter 9

Sound and Electromagnetic WavesSection 9-1

Sound and light are the 2 main ways we receive information about the world.

Sound

Recall: sound waves are mechanical (they require a medium); they reflect, refract and diffract.

To produce sound waves, vibrations need to be much faster than they are in water waves and springs (we can hear sounds ranging from 20Hz to 20000Hz).

Sound waves are longitudinal (medium particles vibrate parallel to direction of wave motion).

They consist of alternating areas of high pressure where medium particles are pushed together (called compressions) and areas of low pressure where medium particles can spread out or expand (called rarefactions).

See Figures 9.2 and 9.3 on Page 377

Loudness (perceived intensity) of sounds is connected to the amplitude of the waves.

Pitch (perceived frequency) of sounds is connected to the frequency of the waves (number of vibrations in medium per second).

Sound quality is what allows listener to distinguish a sound and it is connected to the simplicity/complexity of the wave.

See Figure 9.5 Page 378

Light

Light is an electromagnetic wave and therefore, it does not require a medium to travel through.

Electromagnetic waves are created by moving electric charges (ex. electrons) repelling and attracting each other creating a force or an electric field that resembles a wave.

Light is a form of energy that can be changed into other forms of energy (i.e. green plants change light into chemical energy).

Light is the visible portion of the whole electromagnetic spectrum. It is the range of electromagnetic waves that stimulate the retina of the eye.

Light waves have a wavelength range of 4.00 × 10-7m (violet) to 7.00 X 10-7m (red).

ROY G BIV (See Fig. 9.7 p.380)

In a vacuum or uniform medium, light travels in a straight line.

When not in a vacuum, the velocity of light depends on the medium it is traveling in.

Sound Waves and Reflection

Echo- reflection of a sound wave Echolocation- some animal species can

generate and interpret ultrasonic pulses that reflect off obstacles and prey to allow the animals to locate an object’s exact location, speed and direction. i.e. dolphins and bats

REFLECTION of LIGHT

We see objects either because they reflect or emit (give off) light. The major source of emitted light is the sun. Most objects reflect some light (i.e. moon, trees).

Like all waves, light obeys the law of reflection (i = r).

Incident rays are all parallel and each reflected ray obeys the law of reflection.

Although a surface may appear smooth, it often has many small rises. These small rises cause the reflected rays to be non-parallel in many directions. When a beam of light strikes most surfaces (rough), it reflects in many directions. This is known as diffuse reflection (Fig. 9.13 and 9.14).

Regular reflection is the reflection of light by a smooth surface allowing all reflected rays to be parallel (Fig. 9.13 and 9.14).

☞Driving on a dry night versus driving on a rainy night???

Do 9.1 Section Review on Page 384 #s 1-7

Speed of WavesSection 9-2

Sound The speed of sound in air (at 0°C and 101 kPa)

is 331 m/s and with each 1°C rise in temperature, that speed increases by .59 m/s.

In general, sound travels faster in solids, slower in liquids and slowest in gases (i.e. sound in H2O is almost 5 times faster than in air). See Table 9.1 pg. 387.

Do Model Problems on Page 387 & 388 Do Practice Problems on Page 390 #s 1-6

Mach Number - ratio of speed of object to speed of sound in air that has same temperature and pressure of air in which object is flying.

i.e. Mach 1 means object is flying at exact same speed of sound.

Light Albert Michelson (US physicist) most

accurately measured the speed of light. He reflected a light beam through a 35 km vacuumed pipe using a system of telescopes and rotating mirrors and measured the time it took for light to make a round trip. The velocity measured was 2.99792458 x 108 m/s. For our purposes, we will use 3.00 X 108 m/s as the value for the speed of light.

We’ve used the following formulas already:

Now, we will simply change the ‘v’ to a ‘c’ for light:

Sound Doppler Effect- the apparent change in the

frequency of a sound when the source of the sound and/or receiver of the sound is/are moving.

The siren is making the exact same sound at all times, but as the source of the sound gets closer to the receiver, the waves have less distance to travel and so the waves get closer together (i.e. wavelength decreases and since speed stays the same, the frequency increases). Think of an ambulance coming toward you with its siren on!

Doppler Effect

Big Bang Theory Sonic Boom- an acoustic pressure wave

caused by an object moving faster than the speed of sound.

Sonic Boom

Light

No object having mass can travel at the speed of light in a vacuum.

Cerenkov radiation- same effect as Doppler Shift for sound. The eerie blue glow due to particles (i.e. electrons) traveling in a medium other than a vacuum at a speed greater than the speed of light.

REFRACTION (focus on light)

Refraction is the bending or changing of direction of light passing from one medium to another medium of a different density at an angle greater than 0°, but not 90°.

When an incident ray is perpendicular to the surface of the new medium (≮i = 0°, then ≮refraction = 0°), the transmitted ray will pass straight into the new medium but its velocity will change.

When an incident ray passes from a less dense to a more dense medium at an angle other than 0°, the transmitted light slows down and is bent toward the normal (≮refraction < ≮i).

When an incident ray passes from a more dense to a less dense medium at an angle other than 0°, the transmitted ray speeds up and is bent away from the normal (≮refraction > ≮i).

Snell’s Law A Dutch scientist, Snell, experimented

with the bending of light as it moves from one medium into another. His findings are summarized by Snell’s Law. (Trigonometry basics)

Snell’s Law- on passing from a vacuum into another medium, a light ray is always refracted or bent in such a way that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant.

From a vacuum:

Not from a vacuum:

The ratio of sine θi to sine θr has a different value for each medium, but it is constant for EACH medium and is called the index of refraction (n). *Table 9.2 p. 397

Index of refraction- the ratio of the speed of light in a vacuum to the speed of light in a particular substance.

Refraction occurs because the speed of light depends on the medium through which it travels. The index of refraction is a measure of the amount that light bends when passing into the medium from a vacuum.

A medium is more optically dense than the incident medium if the speed of light is lower in it than it was in the incident medium.

Do Model Problem on Page 399 Do Practice Problems Page 400 #s 7-9

Do Model Problem Page 401 Do Practice Problems Page 405 #s 10 &

11

Other notable refraction effects:

Mirages (apparent wet spots on road in summer)

Apparent shift in object position in liquids (spoon in water appears bent)

Extended daylight after sun has set

Total Internal Reflection When a ray of light passes from a

more dense to a less dense medium, it bends away from the normal making the angle of refraction greater than the angle of incidence.

At a certain angle of incidence the refracted ray makes a right angle with the normal and comes out along the surface between the two mediums.

Fig. 9.30 pg. 407

Critical Angle (θc) - The minimum angle of incidence that causes the refracted ray to lie along the boundary between the two mediums (so θrefraction is 90°).

Critical angles are unique for different substances

where ni is the index of refraction for the substance

If a light ray traveling in a more dense medium reaches the boundary between that medium and a less dense medium at an angle of incidence greater than the critical angle for that substance, then it does not leave the original substance but is reflected back into it. This is known as total internal reflection.There is no refracted ray!

Do Model Problem on Page 409 Do Practice Problems on Page 410 #s 12-

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Total internal reflection is used in many optical instruments such as microscopes, bike reflectors, and telescopes.

In fiber optics, flexible rods are made of very fine strands of glass. Light can be passed down the rod even when it is bent. Whenever light strikes the internal surface of the strands it is totally reflected and continues down the rod.

Fiber optics is used to transmit information (i.e. telephone, radio, and TV signals) because the speed of light is much faster than conventional methods of transmitting this information.

Read pgs. 411-413

Dispersion of Light

In a vacuum, all light waves, regardless of wavelength, travel at 3.00 X 108 m/s, but in other mediums light waves travel more slowly and different wavelengths travel at different speeds. Therefore, the index of refraction of a substance (how much it bends light) depends on the wavelength of light striking it.

Recall that white light consists of 7 different colors ...

ROY G BIV. When white light falls on a prism, the different wavelengths of light bend (refract) to different degrees causing the colors to disperse or separate into a spectrum. Light can be dispersed by glass, diamonds, or water droplets in the air (rainbow).

In most materials, red light travels fastest and therefore, has the smallest index of refraction (bent least). Violet light travels slowest and therefore, has the largest index of refraction (bent most).