magnetic waves? Spectrum 12.3 Radio Communication€¦ · wavelengths of the emitted waves become...

BIG Idea

Science Journal

Electromagnetic waves cantransfer energy through matterand space.

12.1 What are electro-magnetic waves?

Electromagneticwaves are transverse wavesthat can be produced byvibrating electric charges.

12.2 The ElectromagneticSpectrum

Each type of elec-tromagnetic wave has a cer-tain range of frequencies andwavelengths.

12.3 Radio CommunicationSignals and infor-

mation can be transmittedusing radio waves.

How's the reception?These giant 25-m dishesaren't picking up TV signals.Instead, they are part of agroup of 27 antennas thatdetect radio waves comingfrom distant stars and galax-ies. Radio waves, likemicrowaves and light waves,are electromagnetic waves.All objects, including you,emit electromagnetic waves.

List six objects around youthat emit light or feel warm.

MAIN Idea

MAIN Idea

MAIN Idea

352Bob Burch/Index Stock Imagery

Electromagnetic WavesMake the following Foldableto help you understandelectromagnetic waves.

Fold a verticalsheet of paper inhalf from top tobottom.

Fold in half fromside to side withthe fold at the top.

Unfold the paperonce. Cut only thefold of the top flapto make two tabs.

Write on the fronttabs as shown.

Identify Questions As you read the chapter,write answers to the questions on the back ofthe appropriate tabs.

STEP 3

STEP 3

STEP 2

STEP 1

Can electromagnetic waveschange materials? You often hear about the danger of the Sun’sultraviolet rays, which can damage the cellsof your skin. When the exposure isn’t toogreat, your cells can repair themselves, buttoo much at one time can cause a painfulsunburn. Repeated overexposure to the Sunover many years can damage cells and causeskin cancer. In the lab below, observe howenergy carried by ultraviolet waves can causechanges in other materials.

1. Cut a sheet of red construction paperin half.

2. Place one piece outside in direct sunlight.Place the other in a shaded location.

3. Keep the construction paper in full sun-light for at least 45 min. If possible, allowit to stay there for 3 h or more before tak-ing it down. Be sure the other pieceremains in the shade.

4. Think Critically In your Science Journal,describe any differences you notice in thetwo pieces of construction paper.Comment on your results.

Start-Up Activities

Preview this chapter’s contentand activities at gpscience.com

How do electromagnetic

waves travel through space?

How do electromagnetic waves transfer

energy to matter?

353353Bob Burch/Index Stock Imagery

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354 CHAPTER 12 Electromagnetic Waves

Waves in SpaceStay calm. Do not panic. As you are reading this sentence, no

matter where you are, you are surrounded by electromagneticwaves. Even though you can’t feel them, some of these waves aretraveling right through your body. They enable you to see. Theymake your skin feel warm. You use electromagnetic waves whenyou watch television, talk on a cordless phone, or prepare pop-corn in a microwave oven.

Sound and Water Waves Waves are produced by some-thing that vibrates, and they carry energy from one place toanother. Look at the sound wave and the water wave in Figure 1.Both waves are moving through matter. The sound wave is mov-ing through air and the water wave through water. These wavestravel because energy is transferred from particle to particle.Without matter to transfer the energy, they cannot move.

Electromagnetic Waves However, electromagnetic wavesdo not require matter to transfer energy. Electromagnetic wavesare made by vibrating electric charges and can travel throughspace where matter is not present. Instead of transferring energyfrom particle to particle, electromagnetic waves travel by trans-ferring energy between vibrating electric and magnetic fields.

What areelectromagnetic waves?

Reading Guide

■ Describe how electric and mag-netic fields form electromagneticwaves.

■ Explain how vibrating chargesproduce electromagnetic waves.

■ Describe properties of electro-magnetic waves.

You, and all the objects and materi-als around you, are radiating electro-magnetic waves.

Review VocabularyHertz: the SI unit of frequency,abbreviated Hz; 1 Hz equals onevibration per second

New Vocabulary

• electromagnetic wave

• radiant energy

• photon

Figure 1 Water waves and soundwaves require matter to movethrough. Energy is transferred fromone particle to the next as thewave travels through the matter.

SECTION 1 What are electromagnetic waves? 355

Electric and Magnetic FieldsWhen you bring a magnet near a metal paper clip, the paper

clip moves toward the magnet and sticks to it. The paper clipmoved because the magnet exerted a force on it. The magnetexerted this force without having to touch the paper clip. Themagnet exerts a force without touching the paper clip becauseall magnets are surrounded by a magnetic field, as shown inFigure 2. Magnetic fields exist around magnets even if the spacearound the magnet contains no matter.

Just as magnets are surrounded by magnetic fields, electriccharges are surrounded by electric fields, also shown in Figure 2.An electric field enables charges to exert forces on each othereven when they are far apart. Just as a magnetic field arounda magnet can exist in empty space, an electric field existsaround an electric charge even if the space around it containsno matter.

Magnetic Fields and MovingCharges Electric charges also canbe surrounded by magnetic fields.An electric current flowing througha wire is surrounded by a magneticfield, as shown in Figure 3. An elec-tric current in a wire is the flow ofelectrons in a single direction. It isthe motion of these electrons thatcreates the magnetic field aroundthe wire. In fact, any movingelectric charge is surrounded bya magnetic field, as well as anelectric field.

Figure 2 Fields enable magnetsand charges to exert forces at adistance. These fields extendthroughout space. Explain how you could detect amagnetic field.

Magnetic field linesMagnetic field lines

A magnetic field surrounds allmagnets.

An electric field surrounds allcharges.

Figure 3 Electrons moving in awire are surrounded by a magneticfield. Describe how you would confirmthat a magnetic field exists arounda current-carrying wire.


Changing Electric and Magnetic Fields A changingmagnetic field creates a changing electric field. For example, ina transformer, changing electric current in the primary coil pro-duces a changing magnetic field. This changing magnetic fieldthen creates a changing electric field in the secondary coil thatproduces current in the coil. The reverse is also true—a chang-ing electric field creates a changing magnetic field.

Making Electromagnetic WavesWaves such as sound waves are produced when something

vibrates. Electromagnetic waves also are produced when some-thing vibrates—an electric charge that moves back and forth.

What produces an electromagnetic wave?

When an electric charge vibrates, the electric field around itchanges. Because the electric charge is in motion, it also has amagnetic field around it. This magnetic field also changes as thecharge vibrates. As a result, the vibrating electric charge is sur-rounded by changing electric and magnetic fields.

How do the vibrating electric and magnetic fields aroundthe charge become a wave that travels through space? Thechanging electric field around the charge creates a changingmagnetic field. This changing magnetic field then creates achanging electric field. This process continues, with the mag-netic and electric fields continually creating each other. Thesevibrating electric and magnetic fields are perpendicular to eachother and travel outward from the moving charge, as shown inFigure 4. Because the electric and magnetic fields vibrate atright angles to the direction the wave travels, an electromagneticwave is a transverse wave.

Figure 4 A vibrating electriccharge creates an electromagneticwave that travels outward in alldirections from the charge.The wave in only one direction isshown here. Determine whether an electro-magnetic wave is a transverse waveor a compressional wave.

InvestigatingElectromagneticWavesProcedure1. Point your television

remote control in differ-ent directions and observewhether it will still controlthe television.

2. Place various materials infront of the infraredreceiver on the televisionand observe whether theremote still will control thetelevision. Some materialsyou might try are glass, abook, your hand, paper,and a metal pan.

Analysis1. Was it necessary for the

remote to be pointingexactly toward the receiverto control the television?Explain.

2. Did the remote continue towork when the variousmaterials were placedbetween it and thereceiver? Explain why or why not.

Directionof travel

Electric field

Magnetic fieldElectron

THE SPEED OF LIGHT IN WATER The speed of light in water is 226,000 km/s. Write thisnumber in scientific notation.

known values and the unknown value

Identify the known values:

The speed of light in water is 226,000 km/s.

Identify the unknown value:

the number 226,000 written in scientific notation

the problem

A number written in scientific notation has the form M � 10N. N is thenumber of places the decimal point in the number has to be moved so thatthe number M that results has only one digit to the left of the decimal point.

Write the number in scientific notation form: 226,000. � 10N

Move the decimal point five places to the left: 2.26000 � 10N

The decimal point was moved 5 places, so N equals 5: 2.26000 � 105

Delete remaining zeroes at the end of the number. 2.26 � 105

your answer

Add zeroes to the end of the number and move the decimal point in the oppo-site direction five places. The result should be the original number.

CHECK

SOLVE

IDENTIFY


Properties of Electromagnetic WavesAll matter contains charged particles that are always in

motion. As a result, all objects emit electromagnetic waves. Thewavelengths of the emitted waves become shorter as the temper-ature of the material increases. As an electromagnetic wavemoves, its electric and magnetic fields encounter objects. Thesevibrating fields can exert forces on charged particles and magneticmaterials, causing them tomove. For example, electro-magnetic waves from the Suncause electrons in your skin tovibrate and gain energy, asshown in Figure 5. The energycarried by an electromagneticwave is called radiant energy.Radiant energy makes a fire feelwarm and enables you to see.

Electromagneticwave

Electrons

Surface of skin

Nucleus

Write the following numbers in scientific notation: 433; 812,000,000; 73,000,000,000; 84,500.

For more practice problems go to page 834, and visit gpscience.com/extra_problems.

Figure 5 As an electromagneticwave strikes your skin, electrons inyour skin gain energy from thevibrating electric and magneticfields.

Mat Jacob/The Image Works



Wave Speed All electromagnetic waves travel at 300,000km/s in the vacuum of space. Because light is an electromag-netic wave, the speed of electromagnetic waves in space is usu-ally called the “speed of light.” The speed of light is nature’sspeed limit—nothing travels faster than the speed of light. Inmatter, the speed of electromagnetic waves depends on thematerial they travel through. Electromagnetic waves usuallytravel the slowest in solids and the fastest in gases. Table 1 liststhe speed of visible light in various materials.

What is the speed of light?

Wavelength and Frequency Like all waves, electromag-netic waves can be described by their wavelength and frequency.The wavelength of an electromagnetic wave is the distance fromone crest to another, as shown in Figure 6.

The frequency of any wave is the number of wavelengthsthat pass a point in 1 s. The frequency of an electromagneticwave also equals the frequency of the vibrating charge that pro-duces the wave. This frequency is the number of vibrations, orback and forth movements, of the charge in one second. Thefrequency and wavelength of electromagnetic waves are related.As the frequency increases, the wavelength becomes smaller.

Waves and ParticlesThe difference between a wave and a particle might seem obvi-

ous—a wave is a disturbance that carries energy, and a particle isa piece of matter. However, in reality the difference is not so clear.

Waves as Particles In 1887, Heinrich Hertz found that byshining light on a metal, electrons were ejected from the metal.

Hertz found that whether ornot electrons were ejecteddepended on the frequency ofthe light and not the ampli-tude. Because the energycarried by a wave depends onits amplitude and not its fre-quency, this result was mysteri-ous. Years later, Albert Einsteinprovided an explanation—electromagnetic waves canbehave as a particle, called aphoton, whose energy dependson the frequency of the waves.

Magnetic field

Electric field

Wavelength

Wavelength

Figure 6 The wavelength of anelectromagnetic wave is the dis-tance between the crests of thevibrating electric field or magneticfield.

Table 1 Speed of Visible Light

Material Speed (km/s)

Vacuum 300,000

Air slightly less than 300,000

Water 226,000

Glass 200,000

Diamond 124,000


Self Check1. Explain why an electromagnetic wave is a transverse

wave and not a compressional wave.

2. Compare the frequency of an electromagnetic wavewith the frequency of the vibrating charge that pro-duces the wave.

3. Describe how electromagnetic waves transfer radiantenergy to matter.

4. Explain why an electromagnetic wave can travelthrough empty space that contains no matter.

5. Think Critically Suppose a moving electric chargewas surrounded only by an electric field. Inferwhether or not a vibrating electric charge wouldproduce an electromagnetic wave.

SummaryMaking Electromagnetic Waves

• Moving electric charges are surrounded by anelectric field and a magnetic field.

• A vibrating electric charge produces anelectromagnetic wave.

• An electromagnetic wave consists of vibratingelectric and magnetic fields that are perpendi-cular to each other and travel outward fromthe vibrating electric charge.

Properties of Electromagnetic Waves

• Electromagnetic waves carry radiant energy.

• In empty space electromagnetic waves travelat 300,000 km/s—the speed of light.

• Electromagnetic waves travel slower in matter,with a speed that depends on the material.

Waves and Particles

• Electromagnetic waves can behave as parti-cles that are called photons.

• In some circumstances, particles, such aselectrons can behave as waves.

6. Calculate Time How many minutes does it take anelectromagnetic wave to travel 150,000,000 km?

7. Use Scientific Notation Calculate the distance anelectromagnetic wave in space would travel in one day. Express your answer in scientific notation.

Particles as Waves Because electromagnetic waves couldbehave as a particle, others wondered whether matter couldbehave as a wave. If a beam of electrons were sprayed at two tinyslits, you might expect that the electrons would strike only thearea behind the slits, like the spray paint in Figure 7. Instead, itwas found that the electrons formed an interference pattern.This type of pattern is produced by waves when they passthrough two slits and interfere with each other, as the waterwaves do in Figure 7. This experiment showed that electrons canbehave like waves. It is now known that all particles, not onlyelectrons, can behave like waves.

Figure 7 When electrons aresent through two narrow slits,they behave as a wave.

gpscience.com/self_check_quiz

Water waves produce an interfer-ence pattern after passing throughtwo openings.

Particles of paint sprayed throughtwo slits coat only the area behindthe slits.

Electrons fired at two closely-spacedopenings form a wave-like interfer-ence pattern.


Radio waves Infrared waves Ultraviolet waves Gamma rays

MicrowavesVisible light

X rays

INCREASING FREQUENCY


A Range of FrequenciesElectromagnetic waves can have a wide variety of frequen-

cies. They might vibrate once each second or trillions of timeseach second. The entire range of electromagnetic wave frequen-cies is known as the electromagnetic spectrum, shown inFigure 8. Various portions of the electromagnetic spectruminteract with matter differently. As a result, they are given differ-ent names. The electromagnetic waves that humans can detectwith their eyes, called visible light, are a small portion of theentire electromagnetic spectrum. However, various devices havebeen developed to detect the other frequencies. For example, theantenna of your radio detects radio waves.

The ElectromagneticSpectrum

Reading Guide

■ Describe the waves in the differ-ent regions of the electromag-netic spectrum.

■ Compare the properties of differ-ent electromagnetic waves.

■ Identify uses for different typesof electromagnetic waves.

Every day, waves in different regionsof the electromagnetic spectrum areused in many ways.

Review Vocabularyspectrum: a continuous sequencearranged by a particular property

New Vocabulary

• radio wave

• microwave

• infrared wave

• visible light

• ultraviolet wave

• X ray

• gamma ray

Figure 8 Electromagneticwaves are described by differentnames depending on their fre-quency and wavelength.

SECTION 2 The Electromagnetic Spectrum 361

Radio WavesStop and look around you. Even though you can’t see them,

radio waves are moving everywhere you look. Some radio wavescarry an audio signal from a radio station to a radio. However,even though these radio waves carry information that a radiouses to create sound, you can’t hear radio waves. You hear asound wave when the compressions and rarefactions the soundwave produces reach your ears. A radio wave does not producecompressions and rarefactions as it travels through air.

Microwaves Radio waves are low-frequency electromag-netic waves with wavelengths longer than about 1 mm. Radiowaves with wavelengths of less than about 30 cm are calledmicrowaves. Microwaves with wavelengths of about 1 cm to 20 cm are widely used for communication, such as for cellulartelephones and satellite signals. You are probably most familiarwith microwaves because of their use in microwave ovens.

What is the difference between a microwave anda radio wave?

Microwave ovens heat food when microwaves interact withwater molecules in food, as shown in Figure 9. Each water mol-ecule is positively charged on one side and negatively charged onthe other side. The vibrating electric field inside a microwaveoven causes water molecules in food to rotate back and forthbillions of times each second. This rotation causes a type offriction between water molecules that generates thermal energy.It is the thermal energy produced by the interactions betweenthe water molecules that causes your food to cook.

Radar Another use for radio waves is to find the position andmovement of objects by a method called radar. Radar stands forRAdio Detecting And Ranging. With radar, radio waves are trans-mitted toward an object. By measuring the time required for thewaves to bounce off the object and return to a receiving antenna,the location of the object can be found. Law enforcement officersuse radar to measure how fast a vehicle is moving. Radar also is usedfor tracking the movement of aircraft, watercraft, and spacecraft.

Figure 9 Microwave ovens useelectromagnetic waves to heatfood.

Normally water moleculesare randomly arranged.

Electromagneticwave

Heating withMicrowavesProcedure1. Obtain two small beakers

or baby-food jars. Place50 mL of dry sand intoeach. To one of the jars,add 20 mL of room-temperature water andstir well.

2. Record the temperature ofthe sand in each jar.

3. Together, microwave bothjars of sand for 10 s andimmediately record thetemperature again.

Analysis1. Compare the initial and

final temperatures of thewet and dry sand.

2. Infer why there was adifference.

Water molecules

The microwaves causethe water molecules toflip back and forth.

Magnetic Resonance Imaging (MRI) In the early 1980s,medical researchers developed a technique called MagneticResonance Imaging, which uses radio waves to help diagnose ill-ness. The patient lies inside a large cylinder, like the one shown inFigure 10. Housed in the cylinder is a powerful magnet, a radiowave emitter, and a radio wave detector. Protons in hydrogen atomsin bones and soft tissue behave like magnets and align with thestrong magnetic field. Energy from radio waves causes some of theprotons to flip their alignment. As the protons flip, they releaseradiant energy. A radio receiver detects this released energy. Theamount of energy a proton releases depends on the type of tissue itis part of. The released energy detected by the radio receiver is usedto create a map of the different tissues. A picture of the inside of thepatient’s body is produced painlessly.

Infrared Waves Most of the warm air in a fireplace moves up the chimney,

yet when you stand in front of a fireplace, you feel the warmthof the blazing fire. Why do you feel the heat? The warmth youfeel is thermal energy transmitted to you by infrared waves,which are a type of electromagnetic wave with wavelengthsbetween about 1 mm and about 750 billionths of a meter.

You use infrared waves every day. A remote control emitsinfrared waves to control your television. A computer uses infraredwaves to read CD-ROMs. In fact, every object emits infraredwaves. Hotter objects emit more infrared waves than coolerobjects emit. The wavelengths emitted also become shorter as thetemperature increases. Infrared detectors can form images ofobjects from the infrared radiation they emit. Infrared sensors onsatellites can produce infrared images that can help identify thevegetation over a region. Figure 11 shows how cities appear differ-ent from surrounding vegetation in infrared satellite imagery.

Figure 10 Magnetic resonanceimaging technology uses radiowaves as an alternative to X-rayimaging.

362

This infrared image of the same area wastaken from a satellite. In this image, vege-tation is red and buildings are gray.

This visible lightimage of the regionaround San FranciscoBay in California wastaken from an aircraftat an altitude of20,000 m.

Figure 11 Infrared images andvisible light images can providedifferent types of information.

(t)Pete Saloutos/The Stock Market/CORBIS, (bl)NASA/GSFC/JPL, MISR and AirMISR Teams, (br)NASA/GSFC/MITI/ERSDAC/JAROS, and USA/Japan ASTER Science Team


Visible Light Visible light is the range of electromagnetic waves that you

can detect with your eyes. Light differs from radio waves andinfrared waves only by its frequency and wavelength. Visiblelight has wavelengths around 750 billionths to 400 billionths ofa meter. Your eyes contain substances that react differently tovarious wavelengths of visible light, so you see different colors.These colors range from short-wavelength blue to long-wave-length red. If all the colors are present, you see the light as white.

Ultraviolet WavesUltraviolet waves are electromagnetic waves with wave-

lengths from about 400 billionths to 10 billionths of a meter.Ultraviolet waves are energetic enough to enter skin cells.Overexposure to ultraviolet rays can cause skin damage andcancer. Most of the ultraviolet radiation that reaches Earth’s sur-face are longer-wavelength UVA rays. The shorter-wavelengthUVB rays cause sunburn, and both UVA and UVB rays cancause skin cancers and skin damage such as wrinkling. Althoughtoo much exposure to the Sun’s ultraviolet waves is damaging,some exposure is healthy. Ultraviolet light striking the skinenables your body to make vitamin D which isneeded for healthy bones and teeth.

Useful UVs A useful property of ultravioletwaves is their ability to kill bacteria on objectssuch as food or medical supplies. When ultra-violet light enters a cell, it damages proteinand DNA molecules. For some single-celledorganisms, damage can mean death, whichcan be a benefit to health. Ultraviolet wavesare also useful because they make some mate-rials fluoresce (floor ES). Fluorescent materialsabsorb ultraviolet waves and reemit the energyas visible light. As shown in Figure 12, policedetectives sometimes use fluorescent powderto show fingerprints when solving crimes.

CT Scans In certain situa-tions, doctors will performa CT scan on a patientinstead of a traditionalX ray. Research to findout more about CT scans.Compare and contrastCT scans with X rays.What are the advantagesand disadvantages of aCT scan? Write a para-graph about your findingsin your Science Journal.

Figure 12 The police detectivein this picture is shining ultravioletlight on a fingerprint dusted withfluorescent powder.

AP/Wide World Photos


Ultraviolet light hitsa chlorofluorocarbon(CFC) molecule,breaking off achlorine atom.

The chlorine atomreacts with an ozone molecule,pulling off an oxygen atom.

The chlorine atom and the oxygen atom join to form achlorine monoxide molecule.

A free oxygen atom pulls the oxygen atom off the chlorine monoxide molecule.

Once free, the chlorineatom reacts with anotherozone molecule.

UV light

Figure 13 The chlorine atoms inCFCs react with ozone high in theatmosphere. This reaction causesozone molecules to break apart.

The Ozone Layer About 20 to 50 km above Earth’s surface inthe stratosphere is a region called the ozone layer. Ozone is amolecule composed of three oxygen atoms. It is continuallybeing formed and destroyed by ultraviolet waves high in theatmosphere. The ozone layer is vital to life on Earth because itabsorbs most of the Sun’s harmful ultraviolet waves. However,over the past few decades the amount of ozone in the ozonelayer has decreased. Averaged globally, the decrease is aboutthree percent, but is greater at higher latitudes.

Why is the ozone layer vital to life on Earth?

The decrease in ozone is caused by thepresence of certain chemicals, such as

CFCs, high in Earth’s atmosphere. CFCs are chemicals calledchlorofluorocarbons that have been widely used in air condi-tioners, refrigerators, and cleaning fluids. When CFC moleculesreach the ozone layer, they react chemically with ozone mole-cules as shown in Figure 13. One chlorine atom from a CFCmolecule can break apart thousands of ozone molecules. As aresult, many countries are reducing the use of CFCs and otherozone-depleting chemicals.


Self Check1. Explain A mug of water is heated in a microwave oven.

Explain why the water gets hotter than the mug.

2. Describe why you can see visible light waves, but notother electromagnetic waves.

3. List the beneficial effects and the harmful effects ofhuman exposure to ultraviolet rays.

4. Identify three objects in a home that produce electro-magnetic waves and describe how the electromagneticwaves are used.

5. Think Critically What could an infrared image of theirhouse reveal to the homeowners?

SummaryRadio Waves and Infrared Waves

• Radio waves are electromagnetic waves withwavelengths longer than about 1 mm.

• Microwaves are radio waves with wave-lengths between about 1 mm and 1 m.

• Infrared waves have wavelengths betweenabout 1 mm and 750 billionths of a meter.

Visible Light and Ultraviolet Waves

• Visible light waves have wavelengths betweenabout 750 and 400 billionths of a meter.

• Ultraviolet waves have wavelengths betweenabout 400 and 40 billionths of a meter.

• Most of the harmful ultraviolet waves emittedby the Sun are absorbed by the ozone layer.

X Rays and Gamma Rays

• X rays and gamma rays are the most energeticelectromagnetic waves.

• Gamma rays have wavelengths less than10 trillionths of a meter and are produced inthe nuclei of atoms.

6. Use Scientific Notation Express the range of wave-lengths corresponding to visible light, ultravioletwaves, and X rays in scientific notation.

7. Convert Units A nanometer, abbreviated nm, equalsone billionth of a meter, or 10�9 meters. Express therange of wavelengths corresponding to visible light, ultraviolet waves and X rays in nanometers.

X Rays and Gamma RaysThe electromagnetic waves with the shortest wavelengths

and highest frequencies are X rays and gamma rays. Both X raysand gamma rays are high energy electromagnetic waves. X rayshave wavelengths between about ten billionths of a meter andten trillionths of a meter. Doctors and dentists use low doses ofX rays to form images of internal organs, bones, and teeth, likethe image shown in Figure 14. X rays also are used in airportscreening devices to examine the contents of luggage.

Electromagnetic waves with wavelengths shorter than about10 trillionths of a meter are gamma rays. These are the highest-energy electromagnetic waves and can penetrate through severalcentimeters of lead. Gamma rays are produced by processes thatoccur in atomic nuclei. Both X rays and gamma rays are used ina technique called radiation therapy to kill diseased cells in thehuman body. A beam of X rays or gamma rays can damage thebiological molecules in living cells, causing both healthy anddiseased cells to die. However, by carefully controlling theamount of X ray or gamma ray radiation received by the dis-eased area, the damage to healthy cells can be reduced.


Figure 14 Bones are moredense than surrounding tissuesand absorb more X rays. The imageof a bone on an X ray is theshadow cast by the bone as X rayspass through the soft tissue.

Telegraph Colour Library/FPG/Getty Images


Communications satellites transmit signals witha narrow beam pointed toward a particulararea of Earth. To detect this signal, receivers aretypically large, parabolic dishes.

Real-World QuestionHow does the shape of a satellite dish improvereception?

Goals■ Make a model of a satellite reflecting dish.■ Observe how the shape of the dish affects

reception.

Materialsflashlight small bowlseveral books *large, metal spoonaluminum foil *Alternate materials

Safety Precautions

Procedure1. Cover one side of a book with aluminum foil.

Be careful not to wrinkle the foil.

2. Line the inside of the bowl with foil, alsokeeping it as smooth as possible.

3. Place some of the books on a table. Put theflashlight on top of the books so that itsbeam of light will shine several centimetersabove and across the table.

4. Hold the foil-covered book on its side at aright angle to the top of the table. The foil-covered side should face the beam of light.

5. Observe the intensity of the light on the foil.

6. Repeat steps 4 and 5, replacing the foil-covered book with the bowl.

Conclude and Apply1. Compare the brightness of the light

reflected from the two surfaces.

2. Explain why the light you see from thecurved surface is brighter.

3. Infer why bowl-shaped dishes are used toreceive signals from satellites.

Compare your conclusions with thoseobserved by other students in your class.For more help, refer to the Science SkillHandbook.


The Shape of

S$tellite Oishes

Parabolicdish

Focal point

Radio TransmissionWhen you listen to the radio, you hear music and words that

are produced at a distant location. The music and words are sentto your radio by radio waves. The metal antenna of your radiodetects radio waves. As the electromagnetic waves pass by yourradio’s antenna, the electrons in the metal vibrate, as shown inFigure 15. These vibrating electrons produce a changing elec-tric current that contains the information about the musicand words. An amplifier boosts the current and sends it tospeakers, causing them to vibrate. The vibrating speakerscreate sound waves that travel to your ears. Your brain inter-prets these sound waves as music and words

Dividing the Radio Spectrum Each radio station isassigned to broadcast at one particular radio frequency.Turning the tuning knob on your radio allows you to selecta particular frequency to listen to. The specific frequencyof the electromagnetic wave that a radio station is assignedis called the carrier wave.

The radio station must do more than simply transmita carrier wave. The station has to send information aboutthe sounds that you are to receive. This informationis sent by modifying the carrier wave. The carrier wave ismodified to carry information in one of two ways, asshown in Figure 16.

SECTION 3 Radio Communication 367

Radio CommunicationReading Guide

■ Explain how modulating carrierwaves enables information to betransmitted by radio waves.

■ Distinguish between amplitudemodulation and frequencymodulation.

■ Identify various ways of commu-nicating using radio waves.

Every day you use radio wavesto communicate.

Review Vocabularymodulate: to vary the amplitudeor frequency of a wave in order totransmit information

New Vocabulary

• carrier wave

• cathode-ray tube

• transceiver

• Global Positioning System (GPS)

Figure 15 Radio waves exert aforce on the electrons in an antenna,causing the electrons to vibrate.

Antenna

Radiowave

Electron

StudiOhio


AM Radio An AM radio station broadcasts information byvarying the amplitude of the carrier wave, as shown inFigure 16. Your radio detects the variations in amplitude of thecarrier wave and produces a changing electric current fromthese variations. The changing electric current makes thespeaker vibrate. AM carrier wave frequencies range from540,000 to 1,600,000 Hz.

FM Radio Electronic signals are transmitted by FM radio sta-tions by varying the frequency of the carrier wave, as inFigure 16. Your radio detects the changes in frequency of the car-rier wave. Because the strength of the FM waves is kept fixed, FMsignals tend to be more clear than AM signals. FM carrier fre-quencies range from 88 million to 108 million Hz. This is muchhigher than AM frequencies, as shown in Figure 17. Figure 18shows how radio signals are broadcast.

Figure 16 A carrier wavebroadcast by a radio station canbe altered in one of two ways totransmit a signal: amplitudemodulation (AM) or frequencymodulation (FM).

Figure 17 Cell phones, TVs, and radios broadcast atfrequencies that range frommore than 500,000 Hz toalmost 1 billion Hz.

Carrier wave Signal

Amplitude modulation Frequency modulation

Freq

uenc

y (H

z)

1 billion

100 million

10 million

1 million

100,000

AM

TV

TVCell phone

FM

(br)PhotoDisc, (others)Mark Burnett

Figure 18

VISUALIZING RADIO BROADCASTS


You flick a switch, turn the dial, and music from yourfavorite radio station fills the room. Although itseems like magic, sounds are transmitted over great

distances by converting sound waves to electromagneticwaves and back again, as shown here.

At the radio station, musical instrumentsand voices create sound waves by causing air molecules to vibrate. Microphones con-vert these sound waves to a varying electric current, or electronic signal.

A

FMWaves

The modified carrier wave is used tovibrate electrons in the station’s antenna.These vibrating electrons create a radiowave that travels out in all directions atthe speed of light.

C

AMWaves

This signal then is added to the station’s carrierwave. If the station is an AM station, the electronicsignal modifies the amplitude of the carrier wave.If the station is a FM station, the electronic signalmodifies the frequency of the carrier wave.

B

The radio wave from the station makes elec-trons in your radio’s antenna vibrate.This createsan electric current. If your radio is tuned to thestation’s frequency, the carrier wave is removedfrom the original electronic signal.This signalthen makes the radio’s speaker vibrate, creatingsound waves that you hear as music.

D

(tl)Stephen Frisch/Stock Boston/PictureQuest, (tr)Mark Richards/PhotoEdit, Inc./PictureQuest, (bl)CORBIS, (bc)Dorling Kindersley, (br)Michael Newman/PhotoEdit, Inc./PictureQuest


Television What would people hundreds of years ago have thought if

they had seen a television? They might seem like magic, but notif you know how they work. Television and radio transmissionsare similar. At the television station, sound and images arechanged into electronic signals. These signals are broadcast bycarrier waves. The audio part of television is sent by FM radiowaves. Information about the color and brightness is sent at thesame time by AM signals.

Cathode-Ray Tubes In many television sets, images are dis-played on a cathode-ray tube (CRT), as shown in Figure 19. Acathode-ray tube is a sealed vacuum tube in which one or morebeams of electrons are produced. The CRT in a color TV pro-duces three electron beams that are focused by a magnetic fieldand strike a coated screen. The screen is speckled with morethan 100,000 rectangular spots that are of three types. One typeglows red, another glows green, and the third type glows bluewhen electrons strike it. The spots are grouped together with ared, green, and blue spot in each group.

An image is created when the three electron beams of theCRT sweep back and forth across the screen. Each electron beamcontrols the brightness of each type of spot, according to theinformation in the video signal from the TV station. By varyingthe brightness of each spot in a group, the three spots togethercan form any color so that you see a full-color image.

What is a cathode-ray tube?

Figure 19 Cathode-ray tubesproduce the images you see ontelevision. The inside surface ofa television screen is covered bygroups of spots that glow red,green, or blue when struck byan electron beam.

Astronomers Do you everlook up at the stars at nightand wonder how they wereformed? With so manystars and so many galaxies,life might be possible onother planets. Researchways that astronomers useelectromagnetic waves toinvestigate the universe.Choose one projectastronomers currently areworking on that interestsyou, and write about it inyour Science Journal.Discuss the benefits of acareer in astronomy.

Nino Mascardi/The Image Bank/Getty Images


Telephones Until about 1950, human operators were needed

to connect many calls between people. Just 20 yearsago you never would have seen someone walkingdown the street talking on a telephone. Today, cellphones are seen everywhere. When you speak into atelephone, a microphone converts sound waves intoan electrical signal. In cell phones, this current isused to create radio waves that are transmitted toand from a microwave tower, as shown in Figure 20.A cell phone uses one radio signal for sending infor-mation to a tower at a base station. It uses anothersignal for receiving information from the base sta-tion. The base stations are several kilometers apart.The area each one covers is called a cell. If you movefrom one cell to another while using a cell phone, anautomated control station transfers your signal tothe new cell.

What are the cells in a cell phone system?

Cordless Telephones Like a cellular telephone, a cordlesstelephone is a transceiver. A transceiver transmits one radio sig-nal and receives another radio signal from a base unit. Havingtwo signals at different frequencies allows you to talk and listenat the same time. Cordless telephones work much like cellphones. With a cordless telephone, however, you must be closeto the base unit. Another drawback is that when someonenearby is using a cordless telephone, you could hear that conver-sation on your phone if the frequencies match. For this reason,many cordless phones have a channel button. This allows you toswitch your call to another frequency.

Pagers Another method of transmitting signals is a pager,which allows messages to be sent to a small radio receiver. Acaller leaves a message at a central terminal by entering a call-back number through a telephone keypad or by entering a textmessage from a computer. At the terminal, the message ischanged into an electronic signal and transmitted by radiowaves. Each pager is given a unique number for identification.This identification number is sent along with the message. Yourpager receives all messages that are transmitted in the area at itsassigned frequency. However, your pager responds only to mes-sages with its particular identification number. Newer pagerscan send data as well as receive them.

Topic: Radio WaveTechnologyVisit gpscience.com for Web linksto information about advances inradio wave technology.

Activity List the advances youfind, and write about the signifi-cance of each one in your ScienceJournal.

Figure 20 The antenna at thetop of a microwave tower receivessignals from nearby cell phones. Determine whether anymicrowave towers are located nearyour school or home. Describe theirlocations.

Photolink/PhotoDisc



Communications Satellites Since satellites were first developed, thousands have been

launched into Earth’s orbit. Many of these, like the one inFigure 21, are used for communication. A station broadcasts ahigh-frequency microwave signal to the satellite. The satellitereceives the signal, amplifies it, and transmits it to a particularregion on Earth. To avoid interference, the frequency broadcast bythe satellite is different than the frequency broadcast from Earth.

Satellite Telephone Systems If you have a mobile tele-phone, you can make a phone call when sailing across the ocean.To call on a mobile telephone, the telephone transmits radiowaves directly to a satellite. The satellite relays the signal to aground station, and the call is passed on to the telephone net-work. Satellite links work well for one-way transmissions, buttwo-way communications can have an annoying delay caused bythe large distance the signals travel to and from the satellite.

Television Satellites The satellite-reception dishes thatyou sometimes see in yards or attached to houses are receiversfor television satellite signals. Satellite television is used as analternative to ground-based transmission. Communicationssatellites use microwaves rather than the longer-wavelengthradio waves used for normal television broadcasts. Short-wavelength microwaves travel more easily through the atmos-phere. The ground receiver dishes are rounded to help focusthe microwaves onto an antenna.

Figure 21 Communicationssatellites, like the one shown here,use solar panels to provide theelectrical energy they need to com-municate with receivers on Earth.The solar panels are the structureson either side of the central bodyof the satellite.

Topic: SatelliteCommunicationVisit gpscience.com for Web linksto information about wayssatellites are used forcommunication.

Activity Write a paragraphdescribing the advantages of plac-ing a communications satellite ina geosynchronus orbit. Include adiagram.

VCG/FPG/Getty Images



Self Check1. Explain the difference between AM and FM radio. Make

a sketch of how a carrier wave is modulated in AM andFM radio.

2. Define a cathode-ray tube, and explain how it is used ina television.

3. Describe what happens if you are talking on a cellphone while riding in a car and you travel from one cellto another cell.

4. Explain some of the uses of a Global PositioningSystem. Why might emergency vehicles all be equippedwith GPS receivers?

5. Think Critically Why do cordless telephones stop work-ing if you move too far from the base unit?

SummaryRadio Transmission

• Radio stations transmit electromagneticwaves that receivers convert to sound waves.

• Each AM radio station is assigned a carrierwave frequency and varies the amplitude ofthe carrier waves to transmit a signal.

• Each FM radio station is assigned a carrierwave frequency and varies the frequency ofthe carrier waves to transmit a signal.

Television

• TV sets use cathode-ray tubes to convert elec-tronic signals from TV stations into bothsound and images.

Telephones

• Telephones contain transceivers that convertsound waves into electrical signals and alsoconvert electrical signals into sound waves.

• Wires, microwave towers, and satellites areused to transmit and receive telephone signals.

Global Positioning System

• The Global Positioning System uses a systemof satellites to determine your exact position.

6. Calculate a Ratio A group of red, green and bluespots on a TV screen is a pixel. A standard TV has460 pixels horizontally and 360 pixels vertically. Ahigh-definition TV has 1,920 horizontal and 1,080vertical pixels. What is the ratio of the number ofpixels in a high-definition TV to the number in a standard TV?


The Global Positioning SystemGetting lost while hiking is not uncommon,

but if you are carrying a Global Positioning Systemreceiver, it is much less likely to happen. TheGlobal Positioning System (GPS) is a system ofsatellites, ground monitoring stations, andreceivers that determine your exact location at orabove Earth’s surface. The 24 satellites necessaryfor 24-hour, around-the-world coverage becamefully operational in 1995. GPS satellites are ownedand operated by the United States Department ofDefense, but the microwave signals they send outcan be used by anyone. As shown in Figure 22, sig-nals from four satellites are needed to determinethe location of an object using a GPS receiver.Today GPS receivers are used in airplanes, ships,cars, and even by hikers. Figure 22 A GPS receiver uses signals from orbit-

ing satellites to determine the receiver’s location.


Use the InternetUse the Internet


Real-World QuestionThe signals from many radio stations broadcasting at different fre-quencies are hitting your radio’s antenna at the same time. When youtune to your favorite station, the electronics inside your radio amplifythe signal at the frequency broadcast by the station. The signal fromyour favorite station is broadcast from a transmission site that may beseveral miles away.

You may have noticed that if you’re listening to a radio station whiledriving in a car, sometimes the station gets fuzzy and you’ll hearanother station at the same time. Sometimes you lose the station com-pletely. How far can you drive before that happens? Does the distancevary depending on the station you listen to? What are the ranges ofradio stations? Form a hypothesis about how far you think a radio sta-tion can transmit? Which type of signal, AM or FM, has a greater range?Form a hypothesis about the range of your favorite radio station.

Make a Plan1. Research what frequencies are used by AM and FM radio stations

in your area and other areas around the country.

2. Determine these stations’ broadcast locations.

3. Determine the broadcast range of radio stations in your area.

4. Observe how frequencies differ. What is the maximum differencebetween frequencies for FM stations in your area? AM stations?

Goals■ Research which fre-

quencies are used bydifferent radio stations.

■ Observe the receptionof your favorite radiostation.

■ Make a chart of yourfindings and communi-cate them to other students.

Data Source

Visit gpscience.com/internet_lab for more infor-mation on radio frequen-cies, different frequencies ofradio stations around thecountry, and the ranges ofAM and FM broadcasts.

RadKo Frequencies

Len Delessio/Index Stock


Follow Your Plan1. Make sure your teacher approves your plan before you

start.

2. Visit the link shown below for links to different radiostations.

3. Compare the different frequencies of the stations andthe locations of the broadcasts.

4. Determine the range of radio stations in your area andthe power of their broadcast signal in watts.

5. Record your data in your Science Journal.

Analyze Your Data1. Make a map of the radio stations in your area. Do the

ranges of AM stations differ from FM stations?

2. Make a map of different radio stations around the coun-try. Do you see any patterns in the frequencies for stationsthat are located near each other?

3. Write a description that compares how close the frequencies of AM stations areand how close the frequencies of FM stations are. Also compare the power oftheir broadcast signals and their ranges.

4. Share your data by posting it at the link shown below.

Conclude and Apply1. Compare your findings to those of your classmates and other data that was

posted at the link shown below. Do all AM stations and FM stations havedifferent ranges?

2. Observe your map of the country. How closecan stations with similar frequencies be? Do AMand FM stations appear to be different in thisrespect?

3. Infer The power of the broadcast signal alsodetermines its range. How does the power(wattage) of the signals affect your analysisof your data?

LAB 375

Find this lab using the link below. Post yourdata in the table provided. Compare yourdata to that of other students. Then combineyour data with theirs and make a map foryour class that shows all of the data.

gpscience.com/internet_lab

Jim Wark/Index Stock


SCIENCEAND

HISTORYSCIENCE

CAN CHANGE THE COURSE OF HISTORY!

Communicate Research the life of Albert Einstein and make atimeline showing important events in his life. Also include on yourtimeline major historical events that occurred during Einstein’slifetime.

For more information, visitgpscience.com/time

Catch a WaveAt age sixteen, Albert Einstein wondered

“What would it be like to ride a beam of light?”He imagined what might happen if he turned ona flashlight while riding a light beam. Becausethe flashlight was already traveling at the speedof light, would light from the flashlight travel attwice the speed of light?

What’s so special?Einstein thought about this problem and in

1905 he published the special theory of relativity.This theory stated that the speed of light meas-

ured by any observerthat moves with aconstant speed alwayswould be the same.The measured speedof light would notdepend on the speedof the observer or onhow fast the sourceof light was moving.Einstein answeredthe question he asked

himself when he was sixteen. He had found theuniversal speed limit that can’t be broken.

It Doesn’t Add UpAccording to Einstein, electromagnetic

waves like light waves behave very differentlyfrom other waves. For example, sound wavesfrom the siren of an ambulance moving towardyou move faster than they would if the ambu-lance were not moving. The speed of theambulance adds to the speed of the soundwaves. However, for light waves, the speed ofa light source doesn’t add to the speed of light.

Very Strange But TrueEinstein’s special theory of relativity makes

other strange predictions. According to this the-ory, no object can travel faster than the speed oflight. Another prediction is that the measuredlength of a moving object is shorter than whenthe object is at rest. Also, moving clocks shouldrun slower than when they are at rest. Thesepredictions have been confirmed by experi-ments. Measurements have shown, for example,that a moving clock does run slower.

Einstein and the Special Theory of Relativity

Riding a Beamof Light

(t)Getty Images, (b)Bettman/CORBIS


What are electromagneticwaves?

1. Electromagnetic waves consist of vibratingelectric and magnetic fields, and are pro-duced by vibrating electric charges.

2. Electromagnetic waves carry radiant energyand can travel through a vacuum orthrough matter.

3. Electromagnetic waves sometimes behavelike particles called photons.

The ElectromagneticSpectrum

1. Electromag-netic waveswith thelongest wave-lengths arecalled radiowaves. Radiowaves have wavelengths greater than about1 mm. Microwaves are radio waves withwavelengths between about 1 m and 1 mm.

2. Infrared waves have wavelengths betweenabout 1 mm and 750 billionths of a meter.Warmer objects emit more infrared wavesthan cooler objects.

3. Visible light rays have wavelengths betweenabout 750 and 400 billionths of a meter.Substances in your eyes react with visiblelight to enable you to see.

4. Ultraviolet waves have frequencies betweenabout 400 and 10 billionths of a meter.Excessive exposure to ultraviolet waves candamage human skin.

5. X rays and gamma rays are high-energyelectromagnetic waves with wavelengthsless than 10 billionths of a meter. X rays areused in medical imaging.

Radio Communication

1. Modulated radiowaves are used oftenfor communication.AM and FM are twoforms of carrierwave modulation.

2. Television signals aretransmitted as acombination of AMand FM waves.

3. Cellular telephones, cordless telephones,and pagers use radio waves to transmitsignals. Communications satellites are usedto relay telephone and television signalsover long distances.

4. The Global Positioning System enablesan accurate position on Earth to bedetermined.

CHAPTER STUDY GUIDE 377gpscience.com/interactive_tutor

Use the Foldable that you made at the begin-ning of this chapter to help you review electromagneticwaves.

(tl)Martin Dohrn/Science Photo Library/Photo Researchers, (cr)Mickey Pfleger/Photo 20-20/Lonely Planet Images, (bl)Joe McDonald/Tom Stack & Assoc.


Complete each statement using the correct wordor words from the vocabulary list above.

1. _________ are the type of electromagneticwaves often used for communication.

2. A remote control uses _________ to com-municate with a television set.

3. Electromagnetic waves carry ________ .

4. If you stay outdoors too long, your skinmight be burned by exposure to _________from the Sun.

5. A radio station broadcasts radio wavescalled _________ that have the specific fre-quency assigned to the station.

6. The image on a television screen is pro-duced by a _________.

7. Transverse waves that are produced by vibrat-ing electric charges and consist of vibratingelectric and magnetic fields are _________.

Choose the word or phrase that best answers thequestion.

8. Which type of electromagnetic wave is themost energetic?A) gamma raysB) ultraviolet wavesC) infrared wavesD) microwaves

9. Electromagnetic waves can behave likewhat type of particle?A) electrons C) photonsB) molecules D) atoms

10. Which type of electromagnetic waveenables skin cells to produce vitamin D?A) visible lightB) ultraviolet wavesC) infrared wavesD) X rays

11. Which of the following describes X rays?A) short wavelength, high frequencyB) short wavelength, low frequencyC) long wavelength, high frequencyD) long wavelength, low frequency

12. Which of the following is changing in anAM radio wave?A) speed C) amplitudeB) frequency D) wavelength

13. Which type of electromagnetic wave haswavelengths greater than about 1 mm?A) X rays C) gamma raysB) radio waves D) ultraviolet waves

14. What is the name of the ability of somematerials to absorb ultraviolet light and re-emit it as visible light?A) modulation C) transmissionB) handoff D) fluorescence

15. Which of these colors of visible light hasthe shortest wavelength?A) blue C) redB) green D) white

16. Which type of electromagnetic wave haswavelengths slightly longer than humanscan see?A) X raysB) ultraviolet wavesC) infrared wavesD) gamma rays

378 CHAPTER REVIEW

carrier wave p. 367cathode-ray tube p. 370electromagnetic wave

p. 354gamma rays p. 365Global Positioning System

p. 373infrared waves p. 362

microwaves p. 361photon p. 358radiant energy p. 357radio waves p. 361transceiver p. 371ultraviolet waves p. 363visible light p. 363X rays p. 365

gpscience.com/vocabulary_puzzlemaker


CHAPTER REVIEW 379gpscience.com/chapter_review

17. Copy and complete the following tableabout the electromagnetic spectrum.

18. Copy and complete the following eventschain about the destruction of ozone mole-cules in the ozone layer by CFC molecules.

19. Explain why X rays are used in medicalimaging.

20. Predict whether an electromagnetic wavewould travel through space if its electricand magnetic fields were not changingwith time. Explain your reasoning.

21. Infer Electromagnetic waves consist ofvibrating electric and magnetic fields. Amagnetic field can make a compass nee-dle. Why doesn’t a compass needle movewhen visible light strikes the compass?

22. Classify Look around your home, school,and community. Make a list of the differ-ent devices that use electromagneticwaves. Beside each device, write the typeof electromagnetic wave the device uses.

23. Form a hypothesis to explain why communi-cations satellites don’t use ultravioletwaves to receive information and transmitsignals to Earth’s surface.

24. Compare the energy of photons correspon-ding to infrared waves with the energy ofphotons corresponding to ultravioletwaves.

25. Determine whether or not all electromag-netic waves always travel at the speed oflight. Explain.

Interpreting Graphics

26. Use Fractions When visible light wavestravel in ethyl alcohol, their speed isthree fourths of the speed of light inair. What is the speed of light in ethylalcohol?

27. Use Scientific Notation The speed of lightin a vacuum has been determined to be299, 792, 458 m/s. Express this numberto four significant digits using scientificnotation.

28. Calculate Wavelength A radio wave has afrequency of 540,000 Hz and travels ata speed of 300,000 km/s. Use the wavespeed equation to calculate the wave-length of the radio wave. Express youranswer in meters.

Uses of Electromagnetic Waves

Type of Examples of HowElectromagnetic Waves Electromagnetic Waves Are Used

radio, TV transmission

Infrared waves

Visible light vision

X rays

destroying harmful cells

CFCs are released into the air.

CFCs release chlorine atoms.

Ozone is changed to oxygen atomsand molecules.


Record your answers on the answer sheetprovided by your teacher or on a sheet of paper.

1. Which of the following produces electro-magnetic waves?A. vibrating chargeB. direct currentC. static chargeD. constant magnetic field

Use the photograph below to answer questions 2 and 3.

2. A television image is produced by threeelectron beams. What device inside a televi-sion set produces the electron beams?A. transceiver C. antennaB. transmitter D. cathode-ray tube

3. What colors are the three types of glowingspots that are combined to form the differ-ent colors in the image on the screen?A. red, yellow, blueB. red, green, blueC. cyan, magenta, yellowD. cyan, magenta, blue

4. Which of the following explains how inter-ference is avoided between the signals com-munications satellites receive and thesignals they broadcast?A. The signals travel at different speeds.B. The signals have different amplitudes.C. The signals have different frequencies.D. The signals are only magnetic.

5. Which of the following people explainedhow light can behave as a particle, called aphoton, whose energy depends on the fre-quency of light?A. Einstein C. NewtonB. Hertz D. Galileo

Use the table below to answer questions 6 and 7.

6. If you arranged the list of electromagneticwaves shown above in order from shortestto longest wavelength, which would be firston the list?A. radio waves C. gamma raysB. X rays D. visible light

7. Which region of the electromagnetic spec-trum listed in the table above includesmicrowaves?A. gamma rays C. ultraviolet wavesB. radio waves D. infrared waves

8. The warmth you feel when you stand infront of a fire is thermal energy transmittedto you by what type of electromagneticwaves?A. X rays C. ultraviolet wavesB. microwaves D. infrared waves

380 STANDARDIZED TEST PRACTICE

Marking on Tests Be sure to ask if it is okay to write on thetest booklet when taking the test, but make sure you mark allanswers on your answer sheet.

Regions of the Electromagnetic Spectrum

Infrared waves Radio waves Gamma rays

X rays Visible light Ultraviolet waves

PhotoDisc

STANDARDIZED TEST PRACTICE 381

Record your answers on the answer sheetprovided by your teacher or on a sheet of paper.

Use the illustrations below to answer questions 9 and 10.

9. The illustration above shows two radiowaves broadcast by a radio station. Whatis the upper, unmodulated wave called?

10. The lower figure shows the same wave thathas been modulated to carry sound infor-mation. What type of modulation does itshow?

11. The frequency of electromagnetic waves ismeasured in what units? What does thisunit mean?

12. Even on a cloudy day, you can get sun-burned outside. However, inside a glassgreenhouse, you won’t get sunburned.Which type of electromagnetic waves willpass through clouds, but not glass?

13. What term refers to the energy carried byan electromagnetic wave?

14. The following sentence is not true: A mag-netic field creates an electric field, and anelectric field creates a magnetic field.Rewrite the sentence so that it is true.

15. Which type of radio station transmitsradio waves that have a higher frequency,AM stations or FM stations?

Record your answers on a sheet of paper.

16. A CD player converts the musical informa-tion on a CD to a varying electric current.Describe how the varying electric currentproduced by a CD player in a radio stationis converted into radio waves.

17. Explain how an electromagnetic wave thatstrikes a material transfers radiant energyto the atoms in the material.

18. How would changing the amount ofozone in the ozone layer affect the amountof the different types of electromagneticwaves emitted by the Sun that reachEarth’s surface?

19. Explain how the cathode-ray tube in a tel-evision is able to produce all the colorsthat you see in an image on a televisionscreen, using just three electron beams.

20. If all atoms contain electric charges, and ifall atoms are constantly in motion, explainwhy all objects should emit electromag-netic waves.

Use the illustration below to answer questions 21 and 22.

21. The illustration above shows microwavesinteracting with water molecules in food.How does the electric field in microwavesaffect water molecules?

22. Describe how thermal energy inside foodis produced by microwaves interactingwith water molecules.

gpscience.com/standardized_test


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