sound and light sound and light

Sound andLight

Sound andLight

C H A P T E R 15

Chapter Preview

488

1 SoundProperties of SoundMusical InstrumentsHearing and the EarUltrasound and Sonar

2 The Nature of LightWaves and ParticlesThe Electromagnetic Spectrum

3 Reflection and ColorReflection of LightMirrorsSeeing Colors

4 Refraction, Lenses, andPrisms

Refraction of Light LensesDispersion and Prisms

Copyright © by Holt, Rinehart and Winston. All rights reserved.

ACTIVITYACTIVITYFocusFocus

Background Imagine that you are walking through a canyon at sunset. As the light of the sun fades, you see the first starsof the night. Your footsteps make an echo that bounces aroundthe canyon walls. As you approach your destination, you hearthe sounds of people talking around a campfire.

Now imagine that you are walking down a street in a big city. You see the flash of neon signs and the colors of streetlights. You hear the sound of cars, and you hear music from a car radio. Through the open door of a restaurant, you hear dishes clanking and people laughing.

Sound and light carry information about the world around us. This chapter will focus on the behavior of sound waves andlight waves. You will learn how sound is produced, how mirrorsand lenses work, how we see and hear, and how sound and lightare used in different applications, ranging from music to medicine.

Activity 1 Stand outside in front of a large wall, and clap yourhands. Do you hear an echo? How much time passes betweenthe time you clap your hands and the time you hear the echo?Use this estimated time to estimate the distance to the wall. Youwill need two other pieces of information: the speed of sound inair, about 340 m/s, and the speed equation, v � d/t.

Activity 2 Find a crosswalk with a crossing signal. Watch as thesignal changes from “Walk” to “Don’t Walk” and back again. Doesthe crossing signal ever produce a sound? If so, why? If not, whywould it be a good idea for the signal to produce a sound?

Pre-Reading Questions1. How do you hear sounds? Are there

sounds you can’t hear?2. Does a reflection of an object look exactly

like the object? Why or why not?3. How high is the speed of light? How

would you measure it?

Whether in a Coloradocanyon or on a busystreet in New York City,the air is filled withsound and light.

489

www.scilinks.orgTopic: Transfer of Sound and Light EnergySciLinks code: HK4140

Copyright © by Holt, Rinehart and Winston.All rights reserved.

Sound> Recognize what factors affect the speed of sound.> Relate loudness and pitch to properties of sound waves.> Explain how harmonics and resonance affect the sound

from musical instruments.> Describe the function of the ear.> Explain how sonar and ultrasound imaging work.

When you listen to your favorite musical group, you hear avariety of sounds. You may hear the steady beat of a drum,

the twang of guitar strings, the wail of a saxophone, chords froma keyboard, or human voices.

Although these sounds all come from different sources, theyare all longitudinal waves produced by vibrating objects. Howdoes a musical instrument or a stereo speaker make sound wavesin the air? What happens when those waves reach your ears?Why does a guitar sound different from a violin?

Properties of SoundWhen a drummer hits a drum, the head of the drum vibrates upand down, as shown in Figure 1A. Each time the drumheadmoves upward, it compresses the air above it. As the head movesback down again, it leaves a small region of air that has a lowerpressure. As this happens over and over, the drumhead creates aseries of compressions and rarefactions in the air, as shown inFigure 1B.

The from a drum are longitudinal waves, inwhich the particles of air vibrate in the same direction the wavetravels. Sound waves are caused by vibrations, and carry energy

through a medium. Sound waves in air spreadout in all directions away from the source.When sound waves from the drum reach yourears, the waves cause your eardrums to vibrate.Other sounds are produced in different waysfrom the sound waves produced by a drum, butin all cases a vibrating object sets the mediumaround it in motion.

sound waves

O B J E C T I V E S

SECTION

1

K E Y T E R M S

sound wavepitchinfrasoundultrasoundresonancesonar

▲

sound wave a longitudinalwave that is caused by vibra-tions and that travels througha material medium

▲

490 C H A P T E R 1 5

Figure 1 The head of a drum vibrates

up and down when it is struck bythe drummer’s hand.

The vibrations of the drum-head create sound waves in the air.

B

A

A

B


The speed of sound depends on the mediumIf you stand a few feet away from a drummer, it may seem thatyou hear the sound from the drum at the same time that thedrummer’s hand strikes the drum head. Sound waves travel veryfast, but not infinitely fast. The speed of sound in air at roomtemperature is about 346 m/s (760 mi/h).

Table 1 shows the speed of sound in various materials and atvarious temperatures. The speed of sound in a particular mediumdepends on how well the particles can transmit the compressionsand rarefactions of sound waves. In a gas, such as air, the speedof sound depends on how often the molecules of the gas collidewith one another. At higher temperatures, the molecules movearound faster and collide more frequently. An increase in tempera-ture of 10ºC increases the speed of sound in a gas by about 6 m/s.

Sound waves travel faster through liquids and solids thanthrough gases. In a liquid or solid the particles are much closertogether than in a gas, so the vibrations are transferred more rap-idly from one particle to the next. However, some solids, such asrubber, dampen vibrations so that sound does not travel well.Materials like rubber can be used for soundproofing.

Loudness is determined by intensityHow do the sound waves change when you increase the volumeon your stereo or television? The loudness of a sound dependspartly on the energy contained in the sound waves. The intensityof a sound wave describes the rate at which a sound wave trans-mits energy through a given area of the medium. Intensitydepends on the amplitude of the sound wave as well as your dis-tance from the source of the waves. Loudness depends on theintensity of the sound wave. The greater the intensity of a sound,the louder the sound will seem.

S O U N D A N D L I G H T 491

Gases Liquids at 25ºC

Air (0ºC) 331 Water 1490

Air (25ºC) 346 Sea water 1530

Air (100ºC) 386 Solids

Helium (0ºC) 972 Copper 3813

Hydrogen (0ºC) 1290 Iron 5000

Oxygen (0ºC) 317 Rubber 54

Speed of Speed ofMedium sound (m/s) Medium sound (m/s)

Table 1 Speed of Sound in Various Mediums

Quick

ACTIVITYACTIVITYQuickQuickQuick

Sound in DifferentMediums

1. Tie a spoon or otherutensil to the middle of a1–2 m length of string.

2. Wrap the loose ends ofthe string around yourindex fingers and placeyour fingers against yourears.

3. Swing the spoon so thatit strikes a tabletop, andcompare the volume andquality of the soundreceived with thosereceived when you listento the sound directlythrough the air.

4. Does sound travel betterthrough the string orthrough the air?

www.scilinks.orgTopic: Properties of

SoundSciLinks code: HK4112


However, a sound with twice the intensity of another sounddoes not seem twice as loud. Humans perceive loudness on a loga-rithmic scale. This means that a sound seems twice as loud whenits intensity is 10 times the intensity of another sound.

The relative intensity of sounds is found by comparing theintensity of a sound with the intensity of the quietest sound aperson can hear, the threshold of hearing. Relative intensity ismeasured in units called decibels, dB. A difference in intensity of10 dB means a sound seems about twice as loud. Figure 2 showssome common sounds and their decibel levels.

The quietest sound a human can hear is 0 dB. A sound of 120 dB is at the threshold of pain. Sounds louder than this canhurt your ears and give you headaches. Extensive exposure tosounds above 120 dB can cause permanent deafness.

Pitch is determined by frequencyThe of a sound is related to the frequency of sound waves.A high-pitched note is made by something vibrating very rapidly,like a violin string or the air in a flute. A low-pitched sound ismade by something vibrating more slowly, like a cello string orthe air in a tuba.

In other words, a high-pitched sound corresponds to a highfrequency, and a low-pitched sound corresponds to a low fre-quency. Trained musicians are capable of detecting subtle differ-ences in frequency, even as slight as a change of 2 Hz.

pitch

492 C H A P T E R 1 5

Figure 2Sound intensity is measured on alogarithmic scale of decibels.

Relative Intensities of Common Sounds

120 dB30 dB 90 dB70 dB50 dB 150 dB0 dB

Whisper

Normal conversation

Nearby jetairplane

Lawnmower

Vacuum cleaner

Threshold of hearing

Threshold of pain

pitch a measure of how highor low a sound is perceived tobe depending on the fre-quency of the sound wave

▲

QuickACTIVITYACTIVITYQuickQuickQuick

Frequency and Pitch1. Hold one end of a flexi-

ble metal or plastic ruleron a desk with abouthalf of the ruler hangingoff the edge. Bend thefree end of the ruler andthen release it. Can youhear a sound?

2. Try changing the positionof the ruler so that lesshangs over the edge.How does that changethe sound produced?


Humans hear sound waves in a limited frequency rangeThe human ear can hear sounds from sources that vibrate asslowly as 20 vibrations per second (20 Hz) and as rapidly as20 000 Hz. Any sound with a frequency below the range of humanhearing is known as any sound with a frequencyabove human hearing range is known as Many ani-mals can hear frequencies of sound outside the range of humanhearing, as shown in Figure 3.

Musical InstrumentsMusical instruments, from deep bassoons to twangy banjos,come in a wide variety of shapes and sizes and produce a widevariety of sounds. But musical instruments can be grouped intoa small number of categories based on how they make sound.Most instruments produce sound through the vibration ofstrings, air columns, or membranes.

Musical instruments rely on standing wavesWhen you pluck the string of a guitar, particles in the string startto vibrate. Waves travel out to the ends of the string, and thenreflect back toward the middle. These vibrations cause a stand-ing wave on the string, as shown in Figure 4. The two ends of thestrings are nodes, and the middle of the string is an antinode.

ultrasound.infrasound;


Elephant

Human

Dog

Ranges of Hearing for Various Mammals

Figure 4Vibrations on a guitar string pro-duce standing waves on the string.These standing waves in turnproduce sound waves in the air.

infrasound slow vibrationsof frequencies lower than20 Hz

ultrasound any sound wavewith frequencies higher than20 000 Hz

▲▲

12 000 Hz

16 Hz

20 000 Hz

20 Hz

46 000 Hz

40 Hz

150 000 Hz

70 Hz

Dolphin

Figure 3Humans can hear sounds rangingfrom 20 Hz to about 20 000 Hz,but many other animals can hearsounds well into the infrasoundand ultrasound ranges.


By placing your finger on the string somewhere along theneck of the guitar, you can change the pitch of the sound. Thishappens because a shorter length of string vibrates more rapidly.In other words, the standing wave has a higher frequency.

Standing waves can exist only at certain wavelengths on astring. The primary standing wave on a vibrating string has awavelength that is twice the length of the string. The frequencyof this wave, which is also the frequency of the string’s vibrations,is called the fundamental frequency.

All musical instruments use standing waves to producesound. In a flute, standing waves are formed in the column of airinside the flute. The wavelength and frequency of the standingwaves can be changed by opening or closing holes in the flutebody, which changes the length of the air column. Standingwaves also form on the head of a drum, as shown in Figure 5.

Harmonics give every instrument a unique soundIf you play notes of the same pitch on a tuning fork and a clari-net, the two notes will sound different from each other. If you lis-ten carefully, you may be able to hear that the clarinet is actuallyproducing sounds at several different pitches, while the tuningfork produces a pure tone of only one pitch.

A tuning fork vibrates only at its fundamental frequency. Theair column in a clarinet, however, vibrates at its fundamental fre-quency and at certain whole-number multiples of that frequency,called harmonics. Figure 6 shows the harmonics present in a tun-ing fork and a clarinet when each sounds the note A-natural.

494 C H A P T E R 1 5

Harmonics Resultant waveform

Rela

tive

inte

nsity

1 3 5 7 92 4 6 8 10

Harmonics Resultant waveform

Rela

tive

inte

nsity

1 3 5 7 92 4 6 8 10

Figure 6The note A-natural on a clarinetsounds different from the samenote on a tuning fork due to therelative intensity of harmonics.

Figure 5Colored dust lies along the nodesof the standing waves on thehead of this drum.

Clarinet

Tuning fork


In the clarinet, several harmonics combine to make a complexwave. Note, however, that this wave still has a primary frequencythat is the same as the frequency of the wave produced by the tun-ing fork. This is the fundamental frequency, which makes the notesound a certain pitch. Every musical instrument has a character-istic sound quality resulting from the mixture of harmonics.

Instruments use resonance to amplify soundWhen you pluck a guitar string, you can feel that the bridge andthe body of the guitar also vibrate. These vibrations, which are aresponse to the vibrating string, are called forced vibrations. Thebody of the guitar is more likely to vibrate at certain specific fre-quencies called natural frequencies.

The sound produced by the guitar will be loudest when theforced vibrations cause the body of the guitar to vibrate at a natu-ral frequency. This effect is called When resonanceoccurs, the sound is amplified because both the string and theguitar itself are vibrating at the same frequency.

resonance.


Materials

How can you amplify the sound of a tuning fork?

✔ tuning forks of various frequencies ✔ various objects made of metal and wood

✔ rubber block for activating forks

resonance a phenomenonthat occurs when two objectsnaturally vibrate at the samefrequency

▲

1. Activate a tuning fork by striking the tongs of thefork against a rubber block.

2. Touch the base of the tuning fork to differentwood or metal objects, as shown in the figure atright. Listen for any changes in the sound of thetuning fork.

3. Activate the fork again, but now try touching theend of the tuning fork to the ends of other tun-ing forks (make sure that the tines of the forksare free to vibrate, not touching anything). Canyou make another tuning fork start vibrating inthis way?

4. If you find two tuning forks that resonate witheach other, try activating one and holding it nearthe tongs of the other one. Can you make thesecond fork vibrate without touching it?

Analysis1. What are some characteristics of the objects

that helped to amplify the sound of the tuningfork in step 2?

2. What is the relationship between the frequen-cies of tuning forks that resonate with eachother in steps 3 and 4?


The natural frequency of an object depends on its shape, size,and mass, as well as the material it is made of. Complex objectssuch as a guitar have many natural frequencies, so they resonatewell at many different pitches. However, some musical instru-ments, such as an electric guitar, do not resonate well and mustbe amplified electronically.

Hearing and the EarThe head of a drum or the strings and body of a guitar vibrate tocreate sound waves in air. But how do you hear these waves andinterpret them as different sounds?

The human ear is a very sensitive organ that senses vibrationsin the air, amplifies them, and then transmits signals to the brain.In some ways, the process of hearing is the reverse of the processby which a drum head makes a sound. In the ear, sound wavescause membranes to vibrate.

Vibrations pass through three regions in the earYour ear is divided into three regions—outer, middle, and inner—as shown in Figure 7. Sound waves are funneled through thefleshy part of your outer ear and down the ear canal. The earcanal ends at the eardrum, a thin, flat piece of tissue.

When sound waves strike the eardrum, they cause theeardrum to vibrate. These vibrations pass from the eardrumthrough the three small bones of the middle ear—known as thehammer, the anvil, and the stirrup. When the vibrations reach thestirrup, the stirrup strikes a membrane at the opening of theinner ear, sending waves through the spiral-shaped cochlea.

Resonance occurs in the inner earThe cochlea contains a long, flexiblemembrane called the basilar membrane.Different parts of the basilar membranevibrate at different natural frequencies.As waves pass through the cochlea, theyresonate with specific parts of the basilarmembrane.

A wave of a particular frequencycauses only a small portion of the basilarmembrane to vibrate. Hair cells near thatpart of the membrane then stimulatenerve fibers that send an impulse to thebrain. The brain interprets this impulseas a sound with a specific frequency.

496 C H A P T E R 1 5

Figure 7 Sound waves are transmitted as vibrations through the ear.Vibrations in the cochlea stimu-late nerves that send impulsesto the brain.

Outerear

StirrupInner earMiddle ear

Cochlea

Hammer

Anvil

Eardrum

www.scilinks.orgTopic: The EarSciLinks code: HK4034


Ultrasound and SonarIf you shout over the edge of a rock canyon, you may hear thesound reflected back to you in an echo. Like all waves, soundwaves can be reflected. The reflection of sound waves can beused to determine distances and to create maps and images.

Sonar is used for underwater location How can a person on a ship measure the distance to the oceanfloor, which may be thousands of meters from the surface of thewater? One way is to use

A sonar system determines distance by measuring the time ittakes for sound waves to be reflected back from a surface. Asonar device on a ship sends a pulse of sound downward, andmeasures the time, t, that it takes for the sound to be reflectedback from the ocean floor. Using the average speed of the soundwaves in water, v, one can calculate the distance, d, by using aform of the speed equation that solves for distance.

d � vt

If a school of fish or a submarine passes under the ship, thesound pulse will be reflected back much sooner.

Ultrasound waves—sound waves with frequencies above20 000 Hz—work particularly well in sonar systems because theycan be focused into narrow beams and can be directed more easi-ly than other sound waves. Bats, like the one in Figure 8, usereflected ultrasound waves to navigate in flight and to locateinsects for food.

Ultrasound imaging is used in medicineThe echoes of very high frequency ultrasoundwaves, between 1 million and 15 million Hz,are used to produce computerized imagescalled sonograms. Using sonograms, doctorscan safely view organs inside the body with-out having to perform surgery. Sonogramscan be used to diagnose problems, to guidesurgical procedures, or even to view unbornfetuses, as shown in Figure 9.

sonar.


Figure 8Bats use ultrasound echoes tonavigate in flight.

Figure 9An image of an unborn fetus canbe generated from reflected ultra-sound waves.

sonar sound navigation andranging, a system that usesacoustic signals and echoreturns to determine thelocation of objects or tocommunicate

▲


Some ultrasound waves are reflected at boundariesAt high frequencies, ultrasound waves can travel through mostmaterials. But some sound waves are reflected when they passfrom one type of material into another. How much sound isreflected depends on the density of the materials at each boundary.The reflected sound waves from different boundary surfaces arecompiled into a sonogram by a computer and displayed on a screen.

The advantage of using sound to see inside the human bodyis that it doesn’t harm living cells as X rays may do. However, tosee details, the wavelengths of the ultrasound must be slightlysmaller than the smallest parts of the object being viewed. Thehigher the frequency of waves in a given medium are, the shorterthe wavelength will be. Sound waves with a frequency of 15 mil-lion Hz have a wavelength of less than 1 mm when they passthrough soft tissue.

498 C H A P T E R 1 5

S E C T I O N 1 R E V I E W

1. Identify two factors that affect the speed of sound.

2. Explain why sound travels faster in water than in air.

3. Distinguish between infrasound and ultrasound waves.

4. Determine which of the following must change when pitchgets higher.a. amplitude d. intensityb. frequency e. speed of the sound wavesc. wavelength

5. Determine which of the following must change when asound gets louder.a. amplitude d. intensityb. frequency e. speed of the sound wavesc. wavelength

6. Explain why the note middle C played on a piano soundsdifferent from the same note played on a violin.

7. Explain why an acoustic guitar generally sounds louder thanan electric guitar without an electronic amplifier.

8. Describe the process through which sound waves in the airare translated into nerve impulses to the brain.

9. Critical Thinking Why are sonograms made with ultrasoundwaves instead of audible sound waves?

10. Creative Thinking Why do most pianos contain a largesounding board underneath the strings? (Hint: The pianowould be harder to hear without it.)

S U M M A R Y

> The speed of sound wavesdepends on temperature,density, and other proper-ties of the medium.

> Pitch is determined by thefrequency of sound waves.

> Infrasound and ultrasoundlie beyond the range ofhuman hearing.

> The loudness of a sounddepends on intensity.Relative intensity is meas-ured in decibels (dB).

> Musical instruments usestanding waves and reso-nance to produce sound.

> The ear converts vibrationsin the air into nerveimpulses to the brain.

> Reflection of sound or ultra-sound waves can be usedto determine distances orto create sonograms.


> Recognize that light has both wave and particle characteristics.> Relate the energy of light to the frequency of electromagnetic

waves.> Describe different parts of the electromagnetic spectrum.> Explain how electromagnetic waves are used in communication,

medicine, and other areas.

The Nature of Light

Most of us see and feel light almost every moment of ourlives, from the first rays of dawn to the warm glow of a

campfire. Even people who cannot see can feel the warmth of thesun on their skin, which is an effect of infrared light. We are veryfamiliar with light, but how much do we understand about whatlight really is?

Waves and ParticlesIt is difficult to describe all of the properties of light with a singlescientific model. The two most commonly used models describelight either as a wave or as a stream of particles.

Light produces interference patterns like water wavesIn 1801, the English scientist Thomas Young devised an experi-ment to test the nature of light. He passed a beam of lightthrough two narrow openings and then onto a screen on theother side. He found that the light produced a striped pattern onthe screen, like the pattern in Figure 10A. This pattern is similarto the pattern caused by water waves interfering in a ripple tank,as shown in Figure 10B.

O B J E C T I V E S

SECTION

2


K E Y T E R M S

photonintensityradar

▲

Figure 10

Two water waves in a ripple tank alsoproduce an interference pattern with lightand dark bands.

BLight passed through two small openings produceslight and dark bands on a screen.A

www.scilinks.orgTopic: Properties of LightSciLinks code: HK4110


Light can be modeled as a waveBecause the light in Young’s experiment produced interferencepatterns, Young concluded that light must consist of waves. Themodel of light as a wave is still used today to explain many of thebasic properties of light and its behavior.

This model describes light as transverse waves that do notrequire a medium in which to travel. Light waves are also calledelectromagnetic waves because they consist of changing electricand magnetic fields. The transverse waves produced by these fieldscan be described by their amplitude, wavelength, and frequency.

The wave model of light explains much of the observed behav-ior of light. For example, light waves may reflect when they meeta mirror, refract when they pass through a lens, or diffract whenthey pass through a narrow opening. Light waves also interferewith one another, and they can even form standing waves.

The wave model of light cannot explain some observationsIn the early part of the 20th century, physicists began to realizethat some observations could not be explained with the wavemodel of light. For example, when light strikes a piece of metal,electrons may fly off the metal’s surface. Experiments show thatin some cases, dim blue light may knock some electrons off ametal plate, while very bright red light cannot knock off any elec-trons, as shown in Figure 11.

According to the wave model, very bright red light shouldhave more energy than dim blue light because the waves in brightlight should have greater amplitude. But this does not explainhow the blue light can knock electrons off the plate while the redlight cannot.

Light can be modeled as a stream of particlesOne way to explain the effects of light striking a metal plate is toassume that the energy of the light is contained in small packets.A packet of blue light carries more energy than a packet of redlight, enough to knock an electron off the plate. Bright red lightcontains many packets, but no single one has enough energy toknock an electron off the plate.

In the particle model of light, these packets are calledand a beam of light is considered to be a stream of

photons. Photons are considered particles, but they are not likeordinary particles of matter. Photons do not have mass; they aremore like little bundles of energy. But unlike the energy in awave, the energy in a photon is located in a particular place.

photons,

500 C H A P T E R 1 5

photon a unit or quantumof light

▲

Figure 11

Bright red light cannot knockelectrons off this metal plate.A

Dim blue light can knockelectrons off the plate. The wavemodel of light cannot explain thiseffect, but the particle model can.

B

e�

e�

e�


The model of light used depends on the situationLight can be modeled as either waves or particles; so which expla-nation is correct? The success of any scientific theory depends onhow well it can explain different observations. Some effects, suchas the interference of light, are more easily explained with thewave model. Other cases, like lightknocking electrons off a metal plate,are explained better by the particlemodel. The particle model also easilyexplains how light can travel acrossempty space without a medium.

Most scientists currently acceptboth the wave model and the particlemodel of light, and use one or the otherdepending on the situation that theyare studying. Some believe that lighthas a “dual nature,” so that it actuallyhas different characteristics dependingon the situation. In many cases, usingeither the wave model or the particlemodel of light gives good results.

The energy of light is proportional to frequencyWhether modeled as a particle or as a wave, light is also a formof energy. Each photon of light can be thought of as carrying asmall amount of energy. The amount of this energy is propor-tional to the frequency of the corresponding electromagneticwave, as shown in Figure 12.

A photon of red light, for example, carries an amount ofenergy that corresponds to the frequency of waves in red light,4.5 � 1014 Hz. A photon with twice as much energy correspondsto a wave with twice the frequency, which lies in the ultravioletrange of the electromagnetic spectrum. Likewise, a photon withhalf as much energy, which would be a photon of infrared light,corresponds to a wave with half the frequency.

The speed of light depends on the mediumIn a vacuum, all light travels at the same speed, called c. Thespeed of light is very large, 3 � 108 m/s (about 186 000 mi/s).Light is the fastest signal in the universe. Nothing can travelfaster than the speed of light.

Light also travels through transparent mediums, such as air,water, and glass. When light passes through a medium, it travelsslower than it does in a vacuum. Table 2 shows the speed of lightin several different mediums.


2.25 � 1014 Hz

4.5 � 1014 Hz

9.0 � 1014 Hz

1.5 � 10–19 J

3.0 � 10–19 J

6.0 � 10–19 J

Wave frequency Photon energy

Figure 12 The energy of photons of light is related to the frequency ofelectromagnetic waves.

Vacuum 2.997925

Air 2.997047

Ice 2.29

Water 2.25

Quartz (SiO2) 2.05

Glass 1.97

Diamond 1.24

Speed of light

Medium (� 108 m/s)

Table 2 Speed of Light inVarious Mediums


The brightness of light depends on intensityYou have probably noticed that it is easier to read near a lampwith a 100 W bulb than near a lamp with a 60 W bulb. That isbecause a 100 W bulb is brighter than a 60 W bulb. The quantitythat measures the amount of light illuminating a surface is called

and it depends on the amount of light—the numberof photons or waves—that passes through a certain area of space.

Like the intensity of sound, the intensity of light from a lightsource decreases as the light spreads out in spherical wave fronts.Imagine a series of spheres centered around a source of light, asshown in Figure 13. As light spreads out from the source, the num-

ber of photons or waves passing through a givenarea on a sphere, say 1 cm2, decreases. An observerfarther from the light source will therefore see thelight as dimmer than will an observer closer to thelight source.

The Electromagnetic SpectrumLight fills the air and space around us. Our eyescan detect light waves ranging from 400 nm (violet light) to 700 nm (red light). But the visiblespectrum is only one small part of the entire electromagnetic spectrum, shown in Figure 14. Welive in a sea of electromagnetic waves, rangingfrom the sun’s ultraviolet light to radio wavestransmitted by television and radio stations.

The electromagnetic spectrum consists of lightat all possible energies, frequencies, and wave-lengths. Although all electromagnetic waves aresimilar in certain ways, each part of the electro-magnetic spectrum also has unique properties.Many modern technologies, from radar guns tocancer treatments, take advantage of the differentproperties of electromagnetic waves.

intensity,

502 C H A P T E R 1 5

Frequency, Hz Wavelength, m

10

10

10

10

10

10

10

10

10

10

10

10

10

10

10

10

10

10

10

10

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

10

10

10

10

1

10

–10

–9

–6

–3

3

Gamma rays

X rays

Ultraviolet

Visible lightInfared

Microwaves

TV, FM

Radio waves

10 –12

10 6

2 m 3 m1 m

Figure 14The electromagnetic spectrumincludes all possible kinds of light.

Figure 13Less light falls on each unitsquare as the distance fromthe source increases.

intensity the rate at whichenergy flows through a givenarea of space

▲


Sunlight contains ultraviolet lightThe invisible light that lies just beyond violet light falls into theultraviolet (UV) portion of the spectrum. Ultraviolet light hashigher energy and shorter wavelengths than visible light does.Nine percent of the energy emitted by the sun is UV light.Because of its high energy, some UV light can pass through thinlayers of clouds, causing you to sunburn even on overcast days.

X rays and gamma rays are used in medicineBeyond the ultraviolet part of the spectrum lie waves known asX rays, which have even higher energy and shorter wavelengthsthan ultraviolet waves. X rays have wavelengths less than 10–8 m.The highest energy electromagnetic waves are gamma rays,which have wavelengths as short as 10–14 m.

An X-ray image at the doctor’s office is made by passingX rays through the body. Most of them pass right through, but afew are absorbed by bones and other tissues. The X rays that passthrough the body to a photographic plate produce an image suchas the one in Figure 15.

X rays are useful tools for doctors, but they can also be dan-gerous. Both X rays and gamma rays have very high energies, sothey may kill living cells or turn them into cancer cells. However,gamma rays can also be used to treat cancer by killing the dis-eased cells.


Figure 15X-ray images are negatives. Darkareas show where the rays passedthrough, while bright areas showdenser structures in the body.

REAL WORLDAPPLICATIONS

REAL WORLD

Sun Protection Short-termexposure to UV light can causesunburn; prolonged or repeatedexposure may lead to skin cancer.To protect your skin, you shouldshield it from UV light wheneveryou are outdoors by covering yourbody with clothing, wearing a hat,and using a sunscreen.

Sunscreen products contain achemical that blocks some or allUV light, preventing it from pen-etrating your skin. The SkinProtection Factor (SPF) of sun-screens varies as shown in thetable at right.

Applying Knowledge1. A friend is taking an antibiotic,

and his doctor tells him to avoidUV light while on the medica-tion. What SPF factor should heuse, and why?

2. You and another friend decideto go hiking on a cloudy day.Your friend claims that she doesnot need any sunscreen be-cause the sun is not out. Whatis wrong with her reasoning?

SPF factor Effect on skin

None Offers no protection from damage by UV

4 to 6 Offers some protection if you tan easily

7 to 8 Offers extra protection but still permits tanning

9 to 10 Offers excellent protection if you burn easily but would still like to get a bit of a tan

15 Offers total protection from burning

22 Totally blocks UV


Infrared light can be felt as warmthElectromagnetic waves with wavelengths slightly longer than redlight fall into the infrared (IR) portion of the spectrum. Infraredlight from the sun, or from a heat lamp, warms you. Infraredlight is used to keep food warm. You might have noticed reddishlamps above food in a cafeteria. The energy provided by theinfrared light is just enough to keep the food hot without contin-uing to cook it.

Devices and photographic film that are sensitive to infraredlight can reveal images of objects like the one in Figure 16. Aninfrared sensor can be used to measure the heat that objects radi-ate and then create images that show temperature variations. Bydetecting infrared radiation, areas of different temperature canbe mapped. Remote sensors on weather satellites that recordinfrared light can track the movement of clouds and record tem-perature changes in the atmosphere.

Microwaves are used in cooking and communicationElectromagnetic waves with wavelengths in the range of cen-timeters, longer than infrared waves, are known as microwaves.The most familiar application of microwaves today is in cooking.

Microwave ovens in the United States use microwaves with afrequency of 2450 MHz (12.2 cm wavelength). Microwaves arereflected by metals and are easily transmitted through air, glass,paper, and plastic. However, water, fat, and sugar all absorbmicrowaves. Microwaves can travel about 3–5 cm into most foods.

As microwaves penetrate deeper into food, they are absorbedalong with their energy. The rapidly changing electric field of themicrowaves causes water and other molecules to vibrate. Thefood warms up as the energy of these vibrations is delivered toother parts of the food.

Microwaves are also used to carry telecommunication sig-nals. Most mobile phones use microwaves to transmit informa-tion, and space probes transmit signals back to Earth withmicrowaves.

Radio waves are used in communications and radarElectromagnetic waves longer than microwaves are classified asradio waves. Radio waves have wavelengths that range fromtenths of a meter to millions of meters. This portion of the electromagnetic spectrum includes TV signals, AM and FM radiosignals, and other radio waves.

Air-traffic control towers at airports use to determinethe locations of aircraft. Antennas at the control tower emit radiowaves, or sometimes microwaves, out in all directions.

radar

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radar radio detection andranging, a system that usesreflected radio waves todetermine the velocity andlocation of objects

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Figure 16An infrared camera reveals thetemperatures of different parts ofan object.

Because microwaves reflectoff the inside walls of amicrowave oven, they mayform standing waves. Foodlying at the antinodes, wherethe vibrations are at a maxi-mum, gets cooked more thanfood lying at the nodes, wherethere are no vibrations. Forthat reason, most microwaveovens rotate food items toensure even heating.


When the signal reaches an airplane, atransmitter on the plane sends anotherradio signal back to the control tower. Thissignal gives the plane’s location and eleva-tion above the ground.

At shorter range, the original signalsent by the antenna may reflect off theplane and back to a receiver at the controltower. A computer then calculates the dis-tance to the plane using the time delaybetween the original signal and thereflected signal. The locations of variousaircraft around the airport are displayed ona screen like the one shown in Figure 17.

Radar is also used by police to monitorthe speed of vehicles. A radar gun fires aradar signal of known frequency at a mov-ing vehicle and then measures the fre-quency of the reflected waves. Because the vehicle is moving, thereflected waves will have a different frequency, according to theDoppler effect. A computer chip converts the difference in fre-quency into a speed and shows the result on a digital display.



1. State one piece of evidence supporting the wave model oflight and one piece of evidence supporting the particlemodel of light.

2. Name the regions of the electromagnetic spectrum from theshortest wavelengths to the longest wavelengths.

3. Determine which photons have greater energy, those associ-ated with microwaves or those associated with visible light.

4. Determine which band of the electromagnetic spectrum hasthe following:a. the lowest frequency c. the greatest energyb. the shortest wavelength d. the least energy

5. Critical Thinking You and a friend are looking at the stars,and you notice two stars close together, one bright and onefairly dim. Your friend comments that the bright star mustemit much more light than the dimmer star. Is he necessari-ly right? Explain your answer.

S U M M A R Y

> Light can be modeled aselectromagnetic waves oras a stream of particlescalled photons.

> The energy of a photon is proportional to the fre-quency of the correspon-ding light wave.

> The speed of light in avacuum, c, is 3.0 � 108 m/s.Light travels more slowly ina medium.

> The electromagnetic spec-trum includes light at allpossible values of energy,frequency, and wavelength.

Figure 17The radar system in an air trafficcontrol tower uses reflected radiowaves to monitor the location andspeed of airplanes.


Reflection and Color> Describe how light reflects off smooth and rough surfaces.> Explain the law of reflection.> Show how mirrors form real and virtual images.> Explain why objects appear to be different colors.> Describe how colors may be added or subtracted.

You may be used to thinking about light bulbs, candles, andthe sun as objects that send light to your eyes. But everything

else that you see, including this textbook, also sends light to youreyes. Otherwise, you would not be able to see them.

Of course, there is a difference between the light from the sunand the light from a book. The sun emits its own light. The lightthat comes from a book is created by the sun or a lamp, thenbounces off the pages of the book to your eyes.

Reflection of LightEvery object reflects some light and absorbs some light. Mirrors,such as those on the solar collector in Figure 18, reflect almost allincoming light. Because mirrors reflect light, it is possible foryou to see an image of yourself in a mirror.

Light can be modeled as a rayIt is useful to use another model for light, the

to describe reflection, refraction, andmany other effects of light at the scale of everydayexperience. A light ray is an imaginary line run-ning in the direction that the light travels. It is thesame as the direction of wave travel in the wavemodel of light or the path of photons in the parti-cle model of light.

Light rays do not represent a full picture of thecomplex nature of light but are a good approxima-tion of light in many cases. The study of light in casesin which light behaves like a ray is called geometricaloptics. Using light rays, one can trace the path oflight in geometrical drawings called ray diagrams.

light ray,

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light rayvirtual imagereal image

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light ray a line in space thatmatches the direction of theflow of radiant energy

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Figure 18This solar collector in the FrenchPyrenees uses mirrors to reflectand focus light into a huge fur-nace, which can reach tempera-tures of 3000˚C.


Rough surfaces reflect light rays in many directionsMany of the surfaces that we see every day, such as paper, wood,cloth, and skin, reflect light but do not appear shiny. When abeam of light is reflected, the path of each light ray in the beamchanges from its initial direction to another direction. If a sur-face is rough, light striking the surface will be reflected at allangles, as shown in Figure 19A. Such reflection of light into ran-dom directions is called diffuse reflection.

Smooth surfaces reflect light rays in one directionWhen light hits a smooth surface, such as a polished mirror, itdoes not reflect diffusely. Instead, all the light hitting a mirrorfrom one direction is reflected together into a single new direc-tion, as shown in Figure 19B.

The new direction of the light rays is related to the old direc-tion in a definite way. The angle of the light rays reflecting off thesurface, called the angle of reflection, is the same as the angle ofthe light rays striking the surface, called the angle of incidence.This is called the law of reflection.

The angle of incidence equals the angle of reflection.

Both of these angles are measured from a line perpendicular tothe surface at the point where the light hits the mirror. This lineis called the normal. Figure 20 is a ray diagram that illustrates thelaw of reflection.


Light rays reflected froma rough surface are reflectedin many directions.

A

Figure 19

Light rays reflected from asmooth surface are reflectedin the same direction.

B

Figure 20 When light hits a smooth surface, the angle of incidence (u) equals the angle of reflection (u′).

Incominglight Normal

Reflecting surface

Reflectedlight

θθ ′

SPACE SCIENCEThere are two primarytypes of telescopes,refracting and reflect-ing. Refracting tele-

scopes use glass lenses to focuslight into an image at the eye-piece; reflecting telescopes usecurved mirrors to focus light.

The lens of a refracting telescope cannot be very large,because the weight of the glasswould cause the lens to bendout of shape. Curved mirrorsare thinner and lighter thanlenses, so they are stable atlarger diameters.

The largest refracting tele-scope, at the Yerkes Observatoryin Wisconsin, has a lens that is 1 m in diameter. The MaunaKea Observatory in Hawaiihouses four of the largestreflecting telescopes. Two ofthem have single mirrors thatare over 8 m in diameter, andtwo use multiple mirrors for atotal diameter of 10 m.

www.scilinks.orgTopic: ReflectionSciLinks code: HK4118


MirrorsWhen you look into a mirror, you see an image of yourself thatappears to be behind the mirror, as in Figure 21A. It is like seeinga twin or copy of yourself standing on the other side of the glass,although flipped from left to right. You also see a whole room, awhole world of space beyond the mirror, even if the mirror isplaced against a wall. How is this possible?

Flat mirrors form virtual images by reflectionThe ray diagram in Figure 21B shows the path of light rays strik-ing a flat mirror. When a light ray is reflected by a flat mirror, theangle it is reflected is equal to the angle of incidence, as describedby the law of reflection. Some of the light rays reflect off the mir-ror into your eyes.

However, your eyes do not know where the light rays havebeen. They simply sense light coming from certain directions,and your brain interprets the light as if it traveled in straight linesfrom an object to your eyes. As a result, you perceive an image ofyourself behind the mirror.

Of course, there is not really a copy of yourself behind themirror. If someone else looked behind the mirror, they would notsee you, an image, or any source of light. The image that you seeresults from the apparent path of the light rays, not an actualpath. An image of this type is called a The virtualimage appears to be as far behind the mirror as you are in frontof the mirror.

virtual image.

MirrorObject Image

A ray diagram shows where the light actually travelsas well as where you perceive that it has come from.B

Figure 21A virtual image appears behind a flat mirror.A

virtual image an imagethat forms at a location fromwhich light rays appear tocome but do not actuallycome

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508 C H A P T E R 1 5 Copyright © by Holt, Rinehart and Winston. All rights reserved.

Curved mirrors can distort imagesIf you have ever been to the “fun house” at a carnival, you mayhave seen a curved mirror like the one in Figure 22. Your imagein a curved mirror does not look exactly like you. Parts of theimage may be spread out, making you look wide or tall. Otherparts may be compressed, making you look thin or short. Howdoes such a mirror work?

Curved mirrors still create images by reflecting light accord-ing to the law of reflection. But because the surface is not flat, theline perpendicular to the mirror (the normal) points in differentdirections for different parts of the mirror.

Where the mirror bulges out, two light rays that start out par-allel are reflected into different directions, making an image thatis stretched out. Mirrors that bulge out are called convex mirrors.

Similarly, parts of the mirror that are indented reflect twoparallel rays in toward one another, making an image that iscompressed. Indented mirrors are called concave mirrors.

Concave mirrors can create real imagesConcave mirrors are used to focus reflected light. A concave mir-ror can form one of two kinds of images. It may form a virtualimage behind the mirror or a in front of the mirror.A real image results when light rays from a single point of anobject are focused onto a single point or small area.

If a piece of paper is placed at the point where the light rayscome together, the real image appears on the paper. If you triedthis with a virtual image, say by placing a piece of paper behinda mirror, you would not see the image on the paper. That is theprimary difference between a real and a virtual image. With areal image, light rays really exist at the point where the imageappears; a virtual image appears to exist in a certain place, butthere are no light rays there.

Telescopes use curved surfaces to focus lightMany reflecting telescopes use curved mirrors to reflect andfocus light from distant stars and planets. Radio telescopes, suchas the one in Figure 23, gather radio waves from extremely distantobjects, such as galaxies and quasars.

Different materials will reflect certain wavelengths of lightand will allow other wavelengths to pass through them. Becauseradio waves reflect off almost any solid surface, these telescopesdo not need to use mirrors. Instead, parallel radio waves bounceoff a curved dish, which focuses the waves onto another, smallercurved surface poised above the dish. The waves are thendirected into a receiver at the center of the dish.

real image


Figure 22A curved mirror produces adistorted image.

real image an image of anobject formed by light raysthat actually come together ata specific location

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Figure 23A radio telescope dish reflects and focuses radio waves into thereceiver at the center of the dish.


Seeing ColorsThe different wavelengths of visible light correspond to many ofthe colors that you perceive. When you see light with a wave-length of about 550 nm, your brain interprets it as green. If thelight comes from the direction of a leaf, then you may think,“That leaf is green.”

A leaf does not emit light on its own; in the darkness of night,you may not be able to see the leaf at all. So where does the greenlight come from?

Objects have color because they reflect certain wavelengthsIf you pass the light from the sun through a prism, the prismseparates the light into a rainbow of colors. White light from thesun actually contains light from all the visible wavelengths of theelectromagnetic spectrum.

When white light strikes a leaf, as shown in Figure 24A, theleaf reflects light with a wavelength of about 550 nm, correspond-ing to the color green. The leaf absorbs light at other wave-lengths. When the light reflected from the leaf enters your eyes,your brain interprets it as green. Transparent objects such ascolor filters work in a similar way. A green filter transmits greenlight and absorbs other colors.

Likewise, the petals of a rose reflect red light and absorbother colors, so the petals appear to be red. If you view a rose andits leaves under red light, as shown in Figure 24B, the petals willstill appear red but the leaves will appear black. Why?

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Figure 24A Rose in White and Red Light

Under red light, thepetals still look red, butthe leaves look blackbecause there is nogreen light for them to reflect.

B

RR

R

RR

RG

GB

V

YO

R

Under whitelight, the petals of arose reflect red light,while the leavesreflect green light.

A


Colors may add or subtract to produce other colorsTelevisions and computer monitors display many different colorsby combining light of the additive primary colors, red, green, andblue. Adding light of two of these colors together can produce thesecondary colors yellow, cyan, and magenta, as shown inFigure 25A. Mixing all three additive primary colors makes white.

In the reverse process, pigments, paints, or filters of the sub-tractive primary colors, yellow, cyan, and magenta, can be com-bined to create red, green, and blue as shown in Figure 25B. Iffilters or pigments of all three colors are combined in equal pro-portions, all the light is absorbed, leaving black. Black is notreally a color at all; it is the absence of color.



1. List three examples of the diffuse reflection of light.

2. Describe the law of reflection in your own words.

3. Draw a diagram to illustrate the law of reflection.

4. Discuss how a plane mirror forms a virtual image.

5. Discuss the difference, in terms of reflection, betweenobjects that appear blue and objects that appear yellow.

6. Explain why a plant may look green in sunlight but blackunder red light.

7. Critical Thinking A friend says that only mirrors and othershiny surfaces reflect light. Explain what is wrong with thisreasoning.

8. Creative Thinking A convex mirror can be used to seearound a corner at the intersection of hallways. Draw a simple ray diagram illustrating how this works.

S U M M A R Y

> Light is reflected when itstrikes a boundary betweentwo different mediums.

> When light reflects off asurface, the angle of reflec-tion equals the angle ofincidence.

> Mirrors form imagesaccording to the law ofreflection.

> The color of an objectdepends on the wave-lengths of light that theobject reflects.

Figure 25Red, green, and blue lights can

combine to produce yellow,magenta, cyan, or white.

Yellow, magenta, and cyan fil-ters can be combined to producered, green, blue, or black.

B

A

A B


Refraction, Lenses, and Prisms> Describe how light is refracted as it passes between mediums.> Explain how fiber optics use total internal reflection.> Explain how converging and diverging lenses work.> Describe the function of the eye.> Describe how prisms disperse light and how rainbows form.

Light travels in a straight line through empty space. But in oureveryday experience, we encounter light passing through vari-

ous mediums, such as the air, windows, a glass of water, or a pairof eyeglasses. Under these circumstances, the direction of a lightwave may be changed by refraction.

Refraction of LightLight waves bend, or refract, when they pass from one mediumto another. If light travels from one transparent medium toanother at any angle other than straight on, the light changesdirection when it meets the boundary, as shown in Figure 26.Light bends when it changes mediums because the speed of lightis different in each medium.

Imagine pushing a lawn mower at an angle from a sidewalkonto grass, as in Figure 27. The wheel that enters the grass firstwill slow down due to friction. If you keep pushing on the lawnmower, the wheel on the grass will act like a moving pivot, andthe lawn mower will turn to a different angle.

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total internal reflection

lensmagnificationprismdispersion

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Figure 26Light refracts when it passes fromone medium into another.

Figure 27This lawn mower changes directionas it passes from the sidewalkonto the grass.


When light moves from a material in which its speed is higherto a material in which its speed is lower, such as from air to glass,the ray is bent toward the normal, as shown in Figure 28A. Thisis like the lawnmower moving from the sidewalk onto the grass.If light moves from a material in which its speed is lower to onein which its speed is higher, the ray is bent away from the nor-mal, as shown in Figure 28B.

Refraction makes objects appear to be in different positionsWhen a cat looks at a fish underwater, the cat perceives the fishas closer than it actually is, as shown in the ray diagram inFigure 29A. On the other hand, when the fish looks at the catabove the surface, the fish perceives the cat as farther than itreally is, as shown in Figure 29B.

The misplaced images that the cat and the fish see are virtualimages like the images that form behind a mirror. The light raysthat pass from the fish to the cat bend away from the normalwhen they pass from water to air. But the cat’s brain doesn’t knowthat. It interprets the light as if it traveled in a straight line, andsees a virtual image. Similarly, the light from the cat to the fishbends toward the normal, again causing the fish to see a virtualimage.

Refraction in the atmosphere creates miragesHave you ever been on a straight road on a hot, dry summer dayand seen what looks like water on the road? If so, then you mayhave seen a mirage. A mirage is a virtual image caused by refrac-tion of light in the atmosphere.

Light travels at slightly different speeds in air of differenttemperatures. Therefore, when light from the sky passes into thelayer of hot air just above the asphalt on a road, it refracts, bend-ing upward away from the road. Because you see an image of thesky coming from the direction of the road, your mind mayassume that there is water on the road causing a reflection.


Air

Normal

Glass

Normal

AirGlass

Figure 28 When the light ray moves from

air into glass, its path is benttoward the normal.

When the light ray passes fromglass into air, its path is bent awayfrom the normal.

B

A

AirWater

Normal

Normal

AirWater

Figure 29

To the cat on the pier, the fish appears to be closer than itreally is.

A

To the fish, the cat seems to befarther from the surface than itactually is.

B

A B


Light can be reflected at the boundary between two transparent mediumsFigure 30 shows four different beams of lightapproaching a boundary between air and water.Three of the beams are refracted as they passfrom one medium to the other. The fourth beamis reflected back into the water.

If the angle at which light rays meet theboundary between two mediums becomes smallenough, the rays will be reflected as if the bound-ary were a mirror. This angle is called the criticalangle, and this type of reflection is called

Fiber optics use total internal reflectionFiber-optic cables are made by fusing bundles of transparentfibers together, as shown in Figure 31A. Light inside a fiber in afiber-optic cable bounces off of the walls of the fiber due to totalinternal reflection, as shown in Figure 31B.

If the fibers are arranged in the same pattern at both ends ofthe cable, the light that enters one end can produce a clear imageat the other end. Fiber-optic cables of that sort are used to pro-duce images of internal organs during surgical procedures.

Because fiber-optic cables can carry many different frequenciesat once, they transmit computer data or signals for telephone callsmore efficiently than standard metal wires. Many long-distancephone calls are now transmitted over optical fibers.

internal reflection.total

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Figure 31 A fiber optic cable consists of

several glass or plastic fibers bun-dled together.

A

Figure 30The refraction or internal reflectionof light depends on the angle atwhich light rays meet the bound-ary between mediums.

total internal reflectionthe complete reflection thattakes place within a substancewhen the angle of incidenceof light striking the surfaceboundary is less than the criti-cal angle

▲

Light is guided along an optic fiber by multiple internalreflections.

B


LensesYou are probably already very familiar with one common appli-cation of the refraction of light: lenses. From cameras to micro-scopes, eyeglasses to the human eye, lenses change the way wesee the world.

Lenses rely on refractionLight traveling at an angle through a flat piece of glass isrefracted twice—once when it enters the glass and again when itreenters the air. The light ray that exits the glass is still parallel tothe original light ray, but it has shifted to one side.

On the other hand, when light passes through a curved pieceof glass, a there is a change in the direction of the lightrays. This is because each light ray strikes the surface of a curvedobject at a slightly different angle.

A converging lens, as shown in Figure 32A, bends light inward.A lens that bends light outward is a diverging lens, as shown inFigure 32B.

A converging lens can create either a virtual image or a realimage, depending on the distance from the lens to the object. Adiverging lens, however, can only create a virtual image.

Lenses can magnify imagesA magnifying glass is a familiar example of a converging lens. Amagnifying glass reveals details that you would not normally beable to see, such as the pistils of the flower in Figure 33. The largeimage of the flower that you see through the lens is a virtualimage. is any change in the size of an imagecompared with the size of the object. Magnification usually pro-duces an image larger than the object, but not always.

Magnification

lens,


lens a transparent object thatrefracts light waves such thatthey converge or diverge tocreate an image

magnification a change inthe size of an image com-pared with the size of anobject

▲▲

When rays of light pass through a converginglens (thicker at the middle), they are bent inward.A When they pass through a diverging lens

(thicker at the ends), they are bent outward.B

Figure 32

Figure 33A magnifying glass makes a largevirtual image of a small object.


If you hold a magnifying glass over a piece of paper in brightsunlight, you can see a real image of the sun on the paper. Byadjusting the height of the lens above the paper, you can focusthe light rays together into a small area, called the focal point. Atthe focal point, the image of the sun may contain enough energyto set the paper on fire.

Microscopes and refracting telescopes use multiple lensesA compound microscope uses multiple lenses to provide greatermagnification than a magnifying glass. Figure 34 illustrates abasic compound microscope. The objective lens first forms alarge real image of the object. The eyepiece then acts like a mag-nifying glass and creates an even larger virtual image that you seewhen you peer through the microscope.

Section 3 explained how reflecting telescopes use curved mir-rors to create images of distant objects such as planets and galax-ies. Refracting telescopes work more like a microscope, focusinglight through several lenses. Light first passes through a largelens at the top of the telescope, then through another lens at theeyepiece. The eyepiece focuses an image onto your eye.

The eye depends on refraction and lensesThe refraction of light by lenses is not just used in microscopesand telescopes. Without refraction, you could not see at all.

The operation of the human eye, shown in Figure 35, is inmany ways similar to that of a simple camera. Light enters acamera through a large lens, which focuses the light into animage on the film at the back of the camera.

Light first enters the eye through a trans-parent tissue called the cornea. The cornea isresponsible for 70 percent of the refraction oflight in the eye. After the cornea, light passesthrough the pupil, a hole in the colorful iris.

From there, light travels through the lens,which is composed of glassy fibers situatedbehind the iris. The curvature of the lensdetermines how much further the lens refractslight. Muscles can adjust the curvature of thelens until an image is focused on the backlayer of the eye, the retina.

The retina is composed of tiny structures,called rods and cones, that are sensitive tolight. When light strikes the rods and cones,signals are sent to the brain where they areinterpreted as images.

Ocular lens

Eyepiece

Objectivelenses

Body tube

Slide withspecimen

Light source

Pupil

Iris

Cornea

Lens

Opticnerve

Retina

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Figure 34A compound microscope usesseveral lens to produce a highlymagnified image.

Figure 35The cornea and lens refract lightonto the retina at the back of the eye.


Cones are concentrated in the center of the retina, while rodsare mostly located on the outer edges. The cones are responsiblefor color vision, but they only respond to bright light. That is whyyou cannot see color in very dim light. The rods are more sensi-tive to dim light, but cannot resolve details very well. That is whyyou can glimpse faint movements or see very dim stars out of thecorners of your eyes.

Dispersion and PrismsA like the one in Figure 36, can separate white light intoits component colors. Water droplets in the air can also do this,producing a rainbow. But why does the light separate into differ-ent colors?

Different colors of light are refracted differentlyAlthough light waves of all wavelengths travel at the same speed(3.0 � 108 m/s) in a vacuum, when a light wave travels through amedium the speed of the light wave does depend on its wave-length. In the visible spectrum, violet light travels the slowest andred light travels the fastest.

Because violet light travels slower than red light, violet lightrefracts more than red light when it passes from one medium toanother. When white light passes from air to the glass in theprism, violet bends the most, red the least, and the rest of the vis-ible spectrum appears in between. This effect, in which light sepa-rates into different colors because of differences in wave speed, iscalled dispersion.

prism,


Figure 36A prism separates white light intoits component colors. Notice thatviolet light is bent more than redlight.

prism in optics, a systemthat consists of two or moreplane surfaces of a transpar-ent solid at an angle witheach other

dispersion in optics, theprocess of separating a wave(such as white light) of differ-ent frequencies into its indi-vidual component waves (thedifferent colors)

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www.scilinks.orgTopic: RefractionSciLinks code: HK4120


Rainbows are caused by dispersion and internal reflectionRainbows, like the one in Figure 37, may form any time waterdroplets are in the air. When sunlight strikes a droplet of water,the light is dispersed into different colors as it passes from the airinto the water. Some of the light then reflects off the back surfaceof the droplet by total internal reflection. The light disperses fur-ther when it passes out of the water back into the air.

When light finally leaves the droplet, violet light emerges atan angle of 40 degrees, red light at 42 degrees, with the other colors in between. We see light from many droplets as arcs ofcolor, forming a rainbow. Red light comes from droplets higherin the air and violet light comes from lower droplets.

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1. Explain why a lawn mower turns when pushed at an anglefrom a sidewalk onto the grass.

2. Draw a ray diagram showing the path of light when it travels from air into glass.

3. Explain how light can bend around corners inside an opticalfiber.

4. Explain how a simple magnifying glass works.

5. Describe the path of light from the time it enters the eye tothe time it reaches the retina.

6. Critical Thinking Which color of visible light travels theslowest through a glass prism?

7. Critical Thinking A spoon partially immersed in a glass ofwater may appear to be in two pieces. Is the image of thespoon in the water a real image or a virtual image?

8. Creative Thinking If light traveled at the same speed in rain-drops as it does in air, could rainbows exist? Explain.

S U M M A R Y

> Light may refract when itpasses from one mediumto another.

> Light rays may also bereflected at a boundarybetween mediums.

> Lenses form real or virtualimages by refraction.

> Converging lenses causelight rays to converge to a point. Diverging lensescause light rays to spreadapart, or diverge.

> A prism disperses whitelight into a color spectrum.

Figure 37Sunlight is dispersed and inter-nally reflected by water droplets to form a rainbow.

40˚42˚

R

R

V

V


M A T H S K I L L S 519

FractionsThe intensity of light at a distance r from a source with power output P is described by thefollowing equation:

intensity � �

What is the intensity of a 100.0 W light bulb at a distance of 5.00 m?

List all given and unknown values.

Given: power (P ), 100.0 W

distance (r), 5.00 m

Unknown: intensity (W/m2)

Write down the equation for intensity.

intensity �

Solve for intensity.

intensity �

intensity �

intensity � � 0.318 W/m2

Because a watt (W) is the amount of power required to do 1 joule (J) of work in 1 s,0.318 J of energy pass through each square meter of area at a distance of 5.00 m fromthe light bulb each second.

Follow the example above to answer the following questions.

1. What is the intensity of the 100.0 W light bulb in the example above at a distance of1.00 m from the light bulb? How many times greater is this intensity than the intensityat 5.00 m?

2. The total power output of the sun is 3.84 � 1026 W. What is the intensity of sunlight ata distance of 1.49 � 1011 m (the average distance of Earth from the sun)?

3. When the sun is directly above the clouds that form the “surface” of Saturn, the inten-sity of sunlight is 15.13 W/m2. What is the distance from the sun to Saturn? How manytimes farther is this than the distance between the sun and Earth?

100.0 W�314 m2

100.0 W��4 � 3.14 � 25 m2

100.0 W��4 � p � (5.00 m)2

3

P�4pr2

2

1

P�4pr2

power��4p(distance)2

Math SkillsMath SkillsMath Skills

PracticePractice


7. Which situation does not involve ultrasound?a. bats navigatingb. a machine generating sonograms of

a fetusc. ships determining the depth of the

ocean floord. a person tuning a piano

8. The wave interaction most important forecholocation is a. reflection. c. diffraction.b. interference. d. resonance.

9. The speed of light a. depends on the medium.b. is fastest in a vacuum.c. is the fastest speed in the universe.d. All of the above

10. Which of the following forms of light hasthe most energy?a. X rays c. infrared lightb. microwaves d. ultraviolet light

11. Light can be modeled as a. electromagnetic waves.b. a stream of particles called photons.c. rays that travel in straight lines.d. All of the above

12. The energy of light is proportional to a. amplitude. c. frequency.b. wavelength. d. the speed of light.

13. Which of the following wavelengths ofvisible light bends the most when passingthrough a prism?a. red c. greenb. yellow d. blue

14. When you look at yourself in a plane mirror,you see aa. real image behind the mirror.b. real image on the surface of the mirror.c. virtual image that appears to be behind

the mirror.d. virtual image that appears to be in front

of the mirror.

Chapter HighlightsBefore you begin, review the summaries of thekey ideas of each section, found at the end ofeach section. The key vocabulary terms arelisted on the first page of each section.

1. All sound waves are a. longitudinal waves.b. transverse waves.c. electromagnetic waves.d. standing waves.

2. The speed of sound depends on a. the temperature of the medium.b. the density of the medium.c. how well the particles of the medium

transfer energy.d. All of the above

3. A sonar device can use the echoes of ultra-sound underwater to find the a. speed of sound.b. depth of the water.c. temperature of the water.d. height of waves on the surface.

4. Relative intensity is measured in unitsabbreviated as a. dB. c. J.b. Hz. d. V.

5. During a thunderstorm, you see lightningbefore you hear thunder because a. the thunder occurs after the lightning.b. the thunder is farther away than the

lightning.c. sound travels faster than light.d. light travels faster than sound.

6. A flat mirror forms an image that is a. smaller than the object. c. virtual.b. larger than the object. d. real.

UNDERSTANDING CONCEPTSUNDERSTANDING CONCEPTS

520 C H A P T E R 1 5

R E V I E WC H A P T E R 15


15. How is the loudness of a sound related toamplitude and intensity?

16. How is the pitch of a sound related to its frequency?

17. Explain how a guitar produces sound. Usethe following terms in your answer: standingwaves, resonance.

18. Why does a clarinet sound different from atuning fork, even when played at the samepitch? Use these terms in your answer: fundamental frequency, harmonics.

19. Describe infrasonic and ultrasonic waves,and explain why humans cannot hear them.

20. Describe the anatomy of the ear, and explainhow the ear senses vibrations. Use the termsouter ear, middle ear, and inner ear.

21. Define sonar and radar. State their differ-ences and their similarities, and describe asituation in which each technique would beuseful.

22. Explain the difference between a virtualimage and a real image. Give an example ofeach type of image.

23. Explain why a leaf may appear green inwhite light but black in red light. Use thefollowing terms in your answer: wavelength,reflection.

24. What is the difference between a converginglens and a diverging lens?

25. Draw a figure that illustrates what happenswhen light rays hit a rough surface and asmooth surface. Explain how your illustra-tion demonstrates diffuse reflection and thelaw of reflection.

26. Graphing As a ship travels across a lake, asonar device on the ship sends out pulses ofultrasound and detects the reflected pulses.The table below gives the ship’s distancefrom the shore and the time for each pulseto return to the ship. Construct a graph ofthe depth of the lake as a function of dis-tance from the shore.

You may use the following for items 27–30:

> the wave speed equation, v � f � l

> a rearranged form of the speed equation, d � vt

> the average speed of sound in water or softtissue, 1500 m/s

> the speed of light, 3.0 � 108 m/s

27. Sound Waves Calculate the wavelength ofultrasound used in medical imaging if thefrequency is 15 MHz.

28. Sonar Calculate the distance to the bottomof a lake when a ship using sonar receivesthe reflection of a pulse in 0.055 s.

USING VOC ABULARYUSING VOC ABULARY BUILDING GR APHING SKILLSBUILDING GR APHING SKILLS

BUILDING MATH SKILLSBUILDING MATH SKILLS


Distance from Time to receiveshore (m) pulse ( � 10–2 s)

100 1.7

120 2.0

140 2.6

150 3.1

170 3.2

200 4.1

220 3.7

250 4.4

270 5.0

300 4.6


DEVELOPING LI FE/WORK SKILLSDEVELOPING LI FE/WORK SKILLS

37. Creative Thinking Imagine laying this page flat on a table, then standing a mirrorupright at the top of the page. Using the lawof reflection, draw the image of each of thefollowing letters of the alphabet in themirror.

A B C F W T

38. Understanding Systems The glass in green-houses is transparent to certain wavelengthsand opaque to others. Researchthis type of glass, and write aparagraph explaining why it isan ideal material for greenhouses.

39. Applying Knowledge Why is white light notdispersed into a spectrum when it passesthrough a flat pane of glass like a window?

40. Interpreting Graphics Examine the chart ofthe electromagnetic spectrum in Figure 14.Which type of electromagnetic wave has theshortest wavelength? Which has the highestfrequency? How does wavelength change asfrequency increases?

41. Teaching Others Your aunt is scheduled foran ultrasound examination of her gall blad-der and she is worried that it will be painful.All she knows is that the examination hassomething to do with sound. How wouldyou explain the procedure to her?

42. Applying Technology Meteorologists useDoppler radar to measure the speed ofapproaching storms and the velocity of theswirling air in tornadoes.Research this application ofradar, and write a short para-graph describing how it works.

29. Electromagnetic Waves Calculate the wave-length of radio waves from an AM radio sta-tion broadcasting at 1200 kHz.

30. Electromagnetic Waves Waves composinggreen light have a wavelength of about 550 nm. What is their frequency?

31. Interpreting Graphics Review Figure 3,which illustrates ranges of frequencies ofsounds that different animals can hear.Which animal on the chart can hear soundswith the highest pitch?

32. Understanding Systems By listening to an orchestra, how can you determine thatthe speed of sound is the same for allfrequencies?

33. Applying Knowledge As you are walkingthrough a park, you see someone blowing awhistle. You can’t hear the whistle, but younotice that several dogs are responding to it.Why can’t you hear the dog whistle?

34. Applying Knowledge When people strain to hear something, they often cup a handaround the outer ear. How does this helpthem hear better? Some animals, such asrabbits and foxes, have very large outer ears.How does this affect their hearing?

35. Thinking Critically Sonar devices on shipsuse a narrow ultrasonic beam for determin-ing depth. A wider beam is used to locatefish. Why?

36. Aquiring and Evaluating Data A guitar hassix strings, each tuned to a different pitch.Research what pitches the strings are nor-mally tuned to and what frequencies corre-spond to each pitch. Then calculate thewavelength of the sound waves that eachstring produces. Assume the speed of soundin air is 340 m/s.

522 C H A P T E R 1 5

R E V I E WC H A P T E R 15

THINKING CR ITIC ALLYTHINKING CR ITIC ALLY

WRITINGS K I L L

WRITINGS K I L L


43. Connection to Earth Science Landsat satel-lites are remote sensing satellites that candetect electromagnetic waves at a variety ofwavelengths to reveal hidden features onEarth. Research the use of Landsat satellitesto view Earth’s surface. What kind of electro-magnetic waves are detected? What featuresdo Landsat images reveal that cannot beseen with visible light?

44. Connection to Health Research the effect ofUV light on skin. Are all wavelengths of UVlight harmful to your skin? What problemscan too much exposure to UV light cause? IsUV light also harmful to your eyes? If so,how can you protect your eyes?

45. Connection to Biology While most peoplecan see all the colors of the spectrum, peo-ple with colorblindness are unable to see atleast one of the primary colors. What part ofthe eye do you think is malfunctioning incolorblind people?

46. Connection to Fine Arts Describe how youcan make red, green, blue, and black paintwith a paint set containing only yellow,magenta, and cyan paint.

47. Connection to Space Science Telescopescan produce images in several differentregions of the electromagnetic spectrum.Research photos of areas of the galaxy takenwith infrared light, microwaves, and radiowaves. What features are revealed byinfrared light that are hidden in visiblelight? What kinds of objects are often studied with radio telescopes?

48. Concept Mapping Copy the unfinished con-cept map below onto a sheet of paper.Complete the map by writing the correctword or phrase in the lettered boxes.

?

INTEGR ATING CONCEPTSINTEGR ATING CONCEPTS


electromagneticwaves

f.

mass

b.

e.d.

ray diagrams

g.

a.

can bemodeled as

that makeup the

which span awide range of

c.

that donot have

that can berepresented in

which haveenergy

proportional towhich travel in

which are

Art Credits: Fig. 1B, Uhl Studios, Inc.; Fig. 2 (graph), Leslie Kell; Fig. 7, Keith Kasnot; Fig. 11, UhlStudios, Inc.; Fig. 12, Leslie Kell; Fig. 13, Boston Graphics; Fig. 14, Boston Graphics; Fig. 27, UhlStudios, Inc.; Fig. 34, Morgan-Cain & Associates; Fig. 35, Keith Kasnot; “Skills Practice Lab”, UhlStudios, Inc.

Photo Credits: Chapter Opener photo of Times Square by Paul Hardy/CORBIS; camper by RonWatts/CORBIS; Fig. 1A, A. Ramey/PhotoEdit; Fig. 2(students speaking, lawnmower, vacuum, dog),Peter Van Steen/HRW; (jet), Peter Gridley/Getty Images/FPG International; (elephant, human),Image Copyright (c)2004 PhotoDisc, Inc.; (dolphin), Doug Perrine/Masterfile; Fig. 4, Tom PantagesPhotography; Fig. 5, Thomas D. Rossing; Fig. 6(clarinet), SuperStock; (tuning fork), SamDudgeon/HRW; “Quick Lab,” Peter Van Steen/HRW; Fig. 8, Benny Odeur/Wildlife Pictures/PeterArnold, Inc.; Fig. 9(t), Telegraph Colour Library/Getty Images/FPG International; (b), SaturnStills/Science Photo Library/Photo Researchers; Fig. 10B, E. R. Degginger/Bruce Coleman, Inc.; Fig. 15, Ron Chapple/Getty Images/FPG International; Fig. 16, E. R. Degginger/AnimalsAnimals/Earth Scenes; Fig. 17, Telegraph Colour Library/Getty Images/FPG International; Fig. 18,Claude Gazuit/Photo Researchers, Inc.; Fig. 21, 22, Peter Van Steen/HRW; Fig. 23, Telegraph ColourLibrary/Getty Images/FPG International; Fig. 24, Peter Van Steen/HRW; Fig. 25A, LeonardLessin/Peter Arnold, Inc.; Fig. 25B, Sam Dudgeon/HRW; Fig. 26, Richard Megna/FundamentalPhotographs; Fig. 30, Ken Kay/Fundamental Photographs; FIg. 30, Guntram Gerst/Peter Arnold, Inc.;Fig. 32, Richard Menga/Fundamental Photographs; Fig. 33, Peter Van Steen/HRW; Fig. 36, SciencePhoto Library/Photo Researchers, Inc.; Fig. 37, Graham French/Masterfile; “Science In Action”(lasers), Zefa Visual Media/Index Stock Imagery, Inc.; (bog man), Sam Dudgeon/HRW; (butterflies),Yves Gentet.

www.scilinks.orgTopic: Microwaves SciLinks code: HK4087


Forming Images with Lenses

� Procedure

Preparing for Your Experiment1. The shape of a lens determines the size, position,

and types of images that it may form. When parallelrays of light from a distant object pass through a con-verging lens, they come together to form an image ata point called the focal point. The distance from thispoint to the lens is called the focal length. In thisexperiment, you will find the focal length of a lens,and then verify this value by forming images, measur-ing distances, and using the lens formula,

� �

where do� object distance, di � image distance, and f � focal length.

2. On a clean sheet of paper, make a table like the oneshown at right.

3. Set up the equipment as illustrated in the figurebelow. Make sure the lens and screen are securelyfastened to the meterstick.

Determining Focal Length4. Stand about 1 m from a window, and point the

meterstick at a tree, parked car, or similar object.Slide the screen holder along the meterstick until aclear image of the distant object forms on the screen.Measure the distance between the lens and thescreen in centimeters. This distance is very close tothe focal length of the lens you are using. Record this value at the top of your data table.

1�f

1�di

1�do

How can you find the focal length of a lens and verify the value?

> Observe images formed by a convexlens.

> Measure the distance of objects andimages from the lens.

> Analyze yourresults to determine the focal lengthof the lens.

cardboard screen, 10 cm � 20 cmconvex lens, 10 cm to 15 cm

focal lengthlens holderlight box with light bulbmeterstickscreen holdersupports for meterstick

USING SCIENTIFIC METHODS

Introduction

Objectives

Materials

524 C H A P T E R 1 5 Copyright © by Holt, Rinehart and Winston. All rights reserved.

Forming Images5. Set up the equipment as illustrated in the

figure at right. Again, make sure that allcomponents are securely fastened.

6. Place the lens more than twice the focallength from the light box. For example, ifthe lens has a focal length of 10 cm, placethe lens 25 or 30 cm from the light.

7. Move the screen along the meterstick until you get a good image. Record the dis-tance from the light to the lens, do, and the distance from the lens to the screen,di, in centimeters as Trial 1 in your data table. Also record the height of the objectand of the image in millimeters. The object in this case may be either the fila-ment of the light bulb or a cut-out shape in the light box.

8. For Trial 2, place the lens exactly twice the focal length from the object. Slide the screen along the stick until a good image is formed, as in step 7. Record thedistances from the screen and the sizes of the object and image as you did instep 7.

9. For Trial 3, place the lens at a distance from the object that is greater than thefocal length but less than twice the focal length. Adjust the screen, and record themeasurements as you did in step 7.

� Analysis1. Perform the calculations needed to complete your data table.

2. How does �d1

o� � �

d1

i� compare with �

1f

� in each of the three trials?

� Conclusions3. If the object distance is greater than the image distance, how will the size of the

image compare with the size of the object?

Focal length Object Image Size of Size of of lens, f: distance, distance, object image_______ cm do (cm) di (cm) � (mm) (mm)

Trial 1

Trial 2

Trial 3

1�f

1�di

1�do

1�di

1�do

S O U N D A N D L I G H T 525Copyright © by Holt, Rinehart and Winston. All rights reserved.

Science inScience inACTIONACTIONACTION

526 S C I E N C E I N A C T I O N

HolographyHave you ever been watching TV or look-ing at a magazine and seen something thatmade you think, “Wait a minute—is thatreal?” Sometimes, special effects andimages made with computers are so lifelikethat you can’t believe your eyes. But eventhe most realistic TV images or photo-graphs can’t fool you into thinking thatthey are real, solid objects. Images on amovie screen have height and width butno depth. Therefore, these images alwaysappear flat. Solid objects, however, arethree-dimensional, which means they haveheight, width, and depth. Holograms arethree-dimensional images so real that manypeople try to reach out and pick them up!

The Science of HologramsTo create a hologram, a person placesan object behind or near a sheet oftransparent holographic film. Laserlight is shown through the film andonto the object. The light reflects offthe object and back onto the film,where it meets the original light beam.The colliding beams create an imagein the light-sensitive film. Like regularphotographic film, the holographicfilm records the light’s wavelength(which determines color) and ampli-tude (which determines intensity, orbrightness). But holographic film alsocaptures the light’s direction. After thefilm is developed and light is shown on the holographic film, tiny reflectorsin the film bounce back light at exactlythe same directions at which it origi-nally came from the object. The resultis a three-dimensional image thatappears to float in space. Amazingly,holographic images are so completethat the viewer can view the imagefrom different angles and see differentsides of the image, just like a solidobject can be viewed.

Laser light helps capture theinformation required to forma three-dimensional imageon holographic film.

“Lindow Man”lived about2300 yearsago. This holo-gram allowsresearchersanywhere inthe world tostudy his form.


> Science and YouScience and You>

527

1. Applying Knowledge Why don’timages on a TV screen or in photo-graphs appear as real as solid objects?

2. Applying Knowledge What two char-acteristics of light does regular photo-graphic film capture?

3. Applying Knowledge What thirdcharacteristic of light captured byholographic film gives the holographicimage depth?

4. Critical Thinking Why do you thinksome people may object to usingholographic images in museums inplace of originals?

5. Critical Thinking Can you think of apractical application for holographicimages not mentioned here? Write ashort paragraph explaining how a par-ticular problem or challenge could besolved by using a hologram.

Putting Holograms to WorkAlthough simple holograms printed onreflective plastic are a common securitydevice on credit cards, holograms that trulyappear three-dimensional are not yet widelyused. But with recent advances in holo-graphic film, high-definition holograms maysoon appear in some unexpected places.Holographic “clones” of art or cultural artifacts may one day be displayed in mu-seums to avoid risk to the originals. Becauseholographic film contains no pigments ordyes, they do not fade over time and cantherefore be used as a permanent three-dimensional record of rare and fragileobjects, such as a specimen of a nearlyextinct insect. The technology also exists to create hologram-like images of movingobjects—including people—that appearto move through thin air.

This full-color hologram isbrighter and more realisticthanks to recent advancesin holographic film andtechniques.

www.scilinks.orgTopic: HolographySciLinks code: HK4068


sound and light sound and light

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