KEY CONCEPT Electromagnetic Waves Have Many Uses.
Page 1 of 8KEY CONCEPTElectromagnetic waveshave many uses.BEFORE, you learnedNOW, you will learn EM waves transfer energythrough fields EM waves have measurableproperties EM waves interact with matter How EM waves differ fromone another How different types of EMwaves are usedVOCABULARYEXPLORE Radio Waveselectromagneticspectrum p. 560radio waves p. 562microwaves p. 563visible light p. 564infrared light p. 564ultraviolet light p. 565x-rays p. 566gamma rays p. 566How can you make radio waves?MATERIALSPROCEDURE1Tape one end of one length of wire to oneend of the battery. Tape one end of thesecond wire to the other end of the battery.2 Wrap the loose end of one of the wirestightly around the handle of the fork. two 25 cm lengthsof copper wire C or D battery electrical tape metal fork portable radio3 Turn on the radio to the AM band and movethe selector past all stations until you reach static.4 Hold the fork close to the radio. Gently pullthe free end of wire across the fork’s prongs.WHAT DO YOU THINK? What happens when you stroke the prongs withthe wire? How does changing the position of the dial affectthe results?EM waves have different frequencies.reminderRemember that frequency isthe number of waves thatpass a given point per second. The shorter thewavelength, the higherthe frequency.It might seem hard to believe that the same form of energy brownsyour toast, brings you broadcast television, and makes the page youare now reading visible. Yet EM waves make each of these eventspossible. The various types of EM waves differ from each other intheir wavelengths and frequencies.The frequency of an EM wave also determines its characteristicsand uses. Higher-frequency EM waves, with more electromagneticvibrations per second, have more energy. Lower-frequency EMwaves, with longer wavelengths, have less energy.Chapter 17: Electromagnetic Waves 559
Page 2 of 8The Electromagnetic SpectrumThe range of all EM frequencies is known as the electromagneticRESOURCE CENTERCLASSZONE.COMspectrum (SPEHK-truhm), or EM spectrum. The spectrum can beLearn more aboutrepresented by a diagram like the one below. On the left are the wavesthe electromagneticwith the longest wavelengths and the lowest frequencies and energies.spectrum.Toward the right, the wavelengths become shorter, and the frequenciesand energies become higher. The diagram also shows different partsof the spectrum: radio waves, microwaves, infrared light, visible light,ultraviolet light, x-rays, and gamma rays.The EM spectrum is a smooth, gradual progression from thelowest frequencies to the highest. Divisions between the differentparts of the spectrum are useful, but not exact. As you can see fromthe diagram below, some of the sections overlap.The Electromagnetic SpectrumFrequency in Hertz (1 hertz 1 13Infrared LightRadio WavesMicrowavesThe amount of infrared light anobject gives off depends on itstemperature. Above, differentcolors indicate different amountsof infrared light.This woman is speaking on the radio.Radio waves are used for radio and television broadcasts. They are also used forcordless phones, garage door openers,alarm systems, and baby monitors.Not all astronomy involves visible light.Telescopes like the one above pick upmicrowaves from space. Microwaves arealso used for radar, cell phones, ovens,and satellite communications.560 Unit 4: Waves, Sound, and Light
Page 3 of 8Measuring EM WavesBecause all EM waves move at the same speed in a vacuum, thefrequency of an EM wave can be determined from its wavelength.EM wavelengths run from about 30 kilometers for the lowest-frequencyradio waves to trillionths of a centimeter for gamma rays. EM wavestravel so quickly that even those with the largest wavelengths havevery high frequencies. For example, a low-energy radio wave with awavelength of 30 kilometers has a frequency of 10,000 cycles per second.SUPPORTING MAIN IDEASWrite details that supportthe main idea that EMwaves form a spectrumbased on frequency.EM wave frequency is measured in hertz (Hz). One hertz equalsone cycle per second. The frequency of the 30-kilometer radio wavementioned above would be 10,000 Hz. Gamma ray frequencies reachtrillions of trillions of hertz.check your reading10141015Why is wavelength all you need to know to calculate EM wavefrequency in a vacuum?101610171018101910201021102210231024Gamma RaysUltraviolet LightX-RaysVisible LightGamma rays can beused to treat illnessesand to create imageslike this one of a person’s thyroid gland.Visible light is the part ofthe EM spectrum that canbe seen with the humaneye. This bird’s colors comefrom different wavelengths.X-rays are useful for showinghard tissues inside the body, suchas bones. To make images likethe one above, x-ray images haveto be displayed using visible light.The researcher in this photograph isusing ultraviolet light in the processof DNA analysis. A chemical in thesamples gives off visible pink lightwhen ultraviolet rays are present.Chapter 17: Electromagnetic Waves 561
Page 4 of 8Radio waves and microwaves have longwavelengths and low frequencies.Radio waves are EM waves that have the longest wavelengths, the lowestfrequencies, and the lowest energies. Radio waves travel easily throughthe atmosphere and many materials. People have developed numeroustechnologies to take advantage of the properties of radio waves.VOCABULARYMake a frame gamediagram for radio wavesand the other types of EMwaves.Radio WavesRadio was the first technology to use EM waves for telecommunication,which is communication over long distances. A radio transmitterconverts sound waves into radio waves and broadcasts them in differentdirections. Radio receivers in many locations pick up the radio wavesand convert them back into sound waves.1 Sound waves enterthe microphone andare converted intoelectrical impulses.AM SignalInformation is encoded inthe signal by varying theradio wave’s amplitude.FM SignalInformation is encoded inthe signal by varying theradio wave’s frequency.2 The electrical impulsesare converted into radiowaves and broadcastby the transmitter.3 The radio wavesreach a radio receiverand are convertedback into sound.Different radio stations broadcast radio waves at different frequencies. To pick up a particular station, you have to tune your radio tothe frequency for that station. The numbers you see on the radio—such as 670 or 99.5—are frequencies.Simply transmitting EM waves at a certain frequency is notenough to send music, words, or other meaningful sounds. To dothat, the radio transmitter must attach information about the soundsto the radio signal. The transmitter attaches the information bymodulating—that is, changing—the waves slightly. Two commonways of modulating radio waves are varying the amplitude of thewaves and varying the frequency of the waves. Amplitude modulationis used for AM radio, and frequency modulation is used for FM radio.You might be surprised to learn that broadcast television also usesradio waves. The picture part of a TV signal is transmitted using an AMsignal. The sound part is transmitted using an FM signal.check your reading562 Unit 4: Waves, Sound, and LightWhat two properties of EM waves are used to attachinformation to radio signals?
Page 5 of 8MicrowavesA type of EM waves called microwaves comes next on the EMspectrum. Microwaves are EM waves with shorter wavelengths, higherfrequencies, and higher energy than other radio waves. Microwaves gettheir name from the fact that their wavelengths are generally shorterthan those of radio waves. Two important technologies that usemicrowaves are radar and cell phones.reading tipAs you read about thedifferent categories of EMwaves, refer to the diagramon pages 560 and 561.The term radar stands for “radio detection and ranging.”Radar came into wide use during World War II (1939–1945) as a wayof detecting aircraft and ships from a distance and estimating theirlocations. Radar works by transmitting microwaves, receiving reflections of the waves from objects the waves strike, and converting thesepatterns into visual images on a screen. Today, radar technology isused to control air traffic at airports, analyze weather conditions, andmeasure the speed of a moving vehicle.RadarRadar led to the invention of the microwave oven. The discoverythat microwaves could be used to cook food was made by accidentwhen microwaves melted a candy bar inside a researcher’s pocket.A cell phone is actually a radio transmitter and receiverthat uses microwaves. Cell phones depend on an overlapping networkof cells, or areas of land several kilometers in diameter. Each cell has atits center a tower that sends and receives microwave signals. The towerconnects cell phones inside the cell to each other or to the regular wirebased telephone system. These two connecting paths are shown below.Cell Phonesmicrowavetower2communication between two cellphones in different cellsmicrowaveswire-based system(landlines)1communicationbetween two cellphones in one cellcellChapter 17: Electromagnetic Waves 563
Page 6 of 8Infrared, visible, and ultraviolet light havemid-range wavelengths and frequencies.is the part of the EM spectrum that human eyes can see.It lies between 1014 Hz and 1015 Hz. We perceive the longest wavelengths of visible light as red and the shortest as violet. This narrowband is very small compared with the rest of the spectrum. In fact,visible light is only about 1/100,000 of the complete EM spectrum.The area below visible light and above microwaves is the infrared partof the EM spectrum. Above visible light is the ultraviolet part of thespectrum. You will read more about visible light in the next section.Visible lightreading tipInfrared means “belowred.” Ultraviolet means“beyond violet.”Infrared LightThe infrared light part of the spectrum consists of EM frequenciesbetween microwaves and visible light. Infrared radiation is the typeof EM wave most often associated with heat. Waves in this range aresometimes called heat rays. Although you cannot see infrared radiation, you can feel it as warmth coming from the Sun, a fire, or aradiator. Infrared lamps are used to provide warmth in bathroomsand to keep food warm after it is cooked. Infrared rays also helpto cook food—for example, in a toaster or over charcoal.The Electromagnetic SpectrumHow can you detect invisible light?PROCEDURE1Find a place that has both bright sunlight and shade, such as a windowsill.Place the white paper in the shade.2 Using the marker, color the bulbs of the thermometers black. Place one ther-mometer on the paper. After three minutes, record the temperature.3 Position the prism so that it shines a bright color spectrum on the whitepaper. Place the thermometers so that one bulb is in the blue area, one inthe red, and one just outside the red, as shown.4 After five minutes, record the three temperatures.WHAT DO YOU THINK? How did the temperature in the shade compare to thetemperature in the light and just outside of it? How might you explain the difference?CHALLENGE How could you modify the experiment to findthe hottest location in the infrared range?564 Unit 4: Waves, Sound, and LightSKILL FOCUSDrawingconclusionsMATERIALS white paperblack marker3 thermometersprismTIME30 minutes
Page 7 of 8Some animals, such as pit viper snakes, can actually seeinfrared light. Normally, human beings cannot see infraredlight. However, infrared scopes and cameras convert infraredradiation into visible wavelengths. They do this by representing different levels of infrared radiation with differentcolors of visible light. This technology can create usefulimages of objects based on the objects’ temperatures.check your readingHow do human beings perceive infrared radiation?Ultraviolet LightThe ultraviolet light part of the EM spectrum consists of frequenciesabove those of visible light and partially below those of x-rays.Because ultraviolet (UV) light has higher frequencies than visiblelight, it also carries more energy. The waves in this range can damageyour skin and eyes. Sunblock and UV-protection sunglasses aredesigned to filter out these frequencies.In this infrared image,warmer areas appear redand orange, while coolerones appear blue, green,and purple.Ultraviolet light has beneficial effects as well. Because it can damagecells, UV light can be used to sterilize medical instruments and food bykilling harmful bacteria. In addition, UV light causes skin cells to produce vitamin D, which is essential to good health. Ultraviolet light canalso be used to treat skin problems and other medical conditions.Like infrared light, ultraviolet light is visible to some animals. Beesand other insects can see higher frequencies than people can. They seenectar guides—marks that show where nectar is located—that peoplecannot see with visible light. The photographs below show how oneflower might look to a person and to a bee.This photograph shows the flower as it appears invisible light.This photograph shows the flower as it might appearto a bee in ultraviolet light. Bees are able to seenectar guides in the UV range.Chapter 17: Electromagnetic Waves 565
Page 8 of 8X-rays and gamma rays have shortwavelengths and high frequencies.At the opposite end of the EM spectrum from radio waves are x-raysand gamma rays. Both have very high frequencies and energies. X-rayshave frequencies from about 1016 Hz to 1021 Hz. Gamma rays havefrequencies from about 1019 Hz to more than 1024 Hz. Like other EMwaves, x-rays and gamma rays are producedX-rays allow us to see inside theby the Sun and by other stars. People havebody.also developed technologies that use theseEM frequencies.X-rays pass easily through the soft tissuesof the body, but many are absorbed bydenser matter such as bone. If photographicfilm is placed behind the body and x-rays areaimed at the film, only the x-rays that passthrough the body will expose the film. Thismakes x-ray images useful for diagnosingbone fractures and finding dense tumors.But too much exposure to x-rays can damagetissue. Even in small doses, repeated exposureto x-rays can cause cancer over time. When you have your teeth x-rayed,you usually wear a vest made out of lead for protection. Lead blockshigh-frequency radiation.Gamma rays have the highest frequencies and energies of any EMwaves. Gamma rays are produced by some radioactive substances as wellas by the Sun and other stars. Gamma rays can penetrate the soft andthe hard tissues of the body, killing normal cells and causing cancercells to develop. If carefully controlled, this destructive power can bebeneficial. Doctors can also use gamma rays to kill cancer cells andfight tumors.KEY CONCEPTSCRITICAL THINKING1. What two properties of EMwaves change from one end ofthe EM spectrum to the other?4. Infer Why do you thinkremote controls for TVs, VCRs,and stereos use infrared lightrather than ultraviolet light?2. Describe two uses formicrowave radiation.3. How are EM waves used indentistry and medicine?566 Unit 4: Waves, Sound, and Light5. Apply For a camera to makeimages of where heat isescaping from a building inwinter, what type of EM wavewould it need to record?CHALLENGE6. Synthesize When a personin a car is talking on a cellphone, and the car movesfrom one cell to another, theconversation continues withoutinterruption. How might thisbe possible?
The Electromagnetic Spectrum The range of all EM frequencies is known as the (SPEHK-truhm), or EM spectrum. The spectrum can be represented by a diagram like the one below. On the left are the waves with the longest wavelengths and the lowest frequencies and energies. To ward the right, the wavelengths become shorter, and the frequencies
Properties of EM Waves Electromagnetic waves are transverse waves Electromagnetic waves travel at the speed of light Because em waves travel at a speed that is precisely the speed of light, light is an electromagnetic wave Electromagnetic waves carry energy as they travel through space, and this energy can be transferred to objects
Waves and Thermodynamics MASTERING PHSICS 3.4 Electromagnetic Waves Conduct investigations to explain and analyse differences between mechanical and electromagnetic waves. Electromagnetic waves Electromagnetic transverse waves are different from transverse matter waves in that they: Can travel through a vacuum.
electromagnetic waves we can see. We see these waves as the colors of the rainbow. Each color has a different wavelength. Red has the longest wavelength and violet has the shortest wavelength. When all the waves are seen together, they make white light. Visible light waves are the only electromagnetic waves we can see.
electromagnetic waves, like radio waves, microwaves, light, and x-rays are examples of transverse waves. Longitudinal waves travel through a medium in a direction parallel to the direction of travel of the wave. Mechanical waves such as sound waves, seismic waves created by earthquakes, and explosions are all examples of longitudinal waves.
Q: What are mechanical waves? A: Waves that require a medium in which to travel. A medium is the _ that waves travel through o Mediums can be solid, liquid, or gas Examples of mechanical waves include sound waves, seismic waves, ocean waves, etc Q: Describe two types of mechanical waves.
Electromagnetic Spectrum: Radio Waves and Microwaves Electromagnetic waves are created when charged particles move, and these waves occur along a spectrum of different wavelengths. Although the electromagnetic spectrum is continuous, scientists divide it into seven sections based on wavelength. Radio waves have very long wavelengths, and
The Electromagnetic Spectrum Light waves aren’t the only kind of electromagnetic waves. In fact, there is an entire spectrum of electromagnetic waves, as shown in Figure 18. The electromagnetic spectrum is the com-plete range of electromagnetic wave frequencies and wavelengths. At one end of the
Electromagnetic Waves Waves that DO NOT NEED matter (medium) to transfer energy Examples: radiation, TV & radio waves, X-rays, microwaves, lasers, energy from the sun, visible light Electromagnetic waves are considered transverse waves because they have simi
