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10.910.9Some Applications of

Electromagnetic WavesRadio and Television CommunicationsMarconi first recognized the potential for transmitting information over long distances,using electromagnetic waves without any direct connection by wires. From his earlydot–dash Morse code pulses evolved our present sophisticated radio and television networks.

Figure 1 shows the typical components of a modern communications transmitter.Sound waves are detected by a microphone and converted into a weak audio signal. Thiselectrical signal is strengthened by an amplifier before passing into a modulator, whereit either modulates the amplitude (in the case of AM) or creates slight perturbations inthe frequency (in FM) of a radio-frequency (RF) carrier signal from the RF oscillator. ThisRF signal wavelength is different for each station.

audiosignal

(amplified)

soundwaves

audiosignal

microphone

modulatedsignal

transmittingantenna

RF signal = carrier

amplifier modulator

RFoscillator

amplifier

Figure 1Transmitter circuit

TV AntennasThe receiving antenna could be aniron core wrapped with wire, along wire, a metal rod, or a multi-element rooftop antenna. Thesignal could also enter your homeby cable, either as range RF or indigital form. Your cable supplierhas already received the signalwith its equipment, thus replacingthe antenna.

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The carrier frequency is the frequency you “tune” on your radio dial. The signal isthen further amplified and supplied to a transmitting antenna, generating the electro-magnetic wave. A television signal is produced in much the same way as audio radio, exceptthat the carrier frequency is mixed with two signals: one for audio and the other forvideo. With both television and audio radio, electrons in the transmitting antenna oscil-late back and forth at the carrier frequency. The accelerating charges produce the elec-tromagnetic wave, which travels at the speed of light to your home receiver.

In the typical receiver shown in Figure 2, the receiving antenna detectsincoming electromagnetic waves. The incoming, oscillating electro-magnetic fields cause free electrons in the conducting material to move,creating a weak electric current in the antenna. The net effect is the pro-duction of a small electrical signal in the antenna, containing a mix-ture of frequencies from many different transmitting stations.

The first task for the receiver is to select a certain carrier frequency,or small range of frequencies, corresponding to a particular station. Theselected RF signal is then amplified and sent into a demodulator, whichseparates the audio signal from the carrier signal. When this audio fre-quency (AF) signal has been successfully separated, it is amplified

loudspeaker

receivingantenna

audiosignal

RF signal

RF tunerand amplifier

detector(demodulation)

AFamplifier

Figure 2Receiver circuit

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and sent to the speaker for conversion into sound waves. In TV transmission, two distinctsignals are demodulated, with the audio signal going to the speaker and the video signalto the picture tube.

In addition to radio and TV bands, frequency bands have been assigned for CitizensBand (CB) radio, ship-to-shore radio, aircraft, police, military, and amateur radio, cel-lular telephone, space and satellite communications, and radar (Figure 3).

InfraredInfrared radiation (IR) occupies the region between microwaves and visible light. Thefrequencies range from about 1.0 � 1011 Hz to about 4 � 1014 Hz. We know that very hotobjects emit electromagnetic radiation. At relatively low temperatures, we can feel theheat—infrared radiation—from an electric stove element. At higher temperatures, theelement begins to glow red, indicating a range of emission in the visible region of thespectrum. At even higher temperatures, such as that of an incandescent light bulb filament,a white glow is observed. We can see that as the temperature of matter increases, the radi-ation it predominantly emits is of higher and higher frequency. But the lower frequenciesare still there. A light bulb, for example, still gives off heat, noticeable even at a distance.

The most common detectors of infrared radiation are photographic film and televisioncameras sensitive to radiation in the infrared range. Full-colour pictures can show minutevariations in the temperature of an object, as indicated by a different colour. For example,infrared photographs of the exterior of a house reveal “hot” spots around the doors andwindows, areas in which heat is leaking out (Figure 4). Heat images of the human bodyreveal areas of infection and locations of tumours. Infrared satellite photographs revealthe type of crops being grown (Figure 5), the population density of urban areas, and thedistribution of acid rain. Reconnaissance photographs of military installations reveal thelocations of airport runways, camouflaged factories, and rocket launch sites.

Laser RadiationConventional light sources are either hot bodies or emitters of some sharply definedsets of wavelengths. The atoms in the tungsten filament of a common light bulb are agi-tated and excited to higher energy levels by high temperatures. Once excited, they emitlight as they return to a lower energy state, over a wide spread of frequencies. In a gas lampsuch as a fluorescent tube, it is the electron current passing through the gas that excitesthe atoms to high energy levels. The atoms give up this excitation energy by radiating itas light waves (see Section 12.4). Spontaneous emission from each single atom takesplace independently of the emission from the others. The overall energy produced by eitherof these conventional light sources—the incandescent lamp or the fluorescent lamp—is a jumble of frequencies from numerous individual atoms. Each emission has its ownphase that changes randomly from moment to moment and point to point. The light fromconventional sources is therefore said to be incoherent.

In contrast, laser light is coherent. The emitted light waves are in phase, with almostall the crests and troughs in step for a substantial time. This coherence arises because thelaser atoms do not emit at random but under stimulation (see Section 12.4 for a moredetailed explanation). The light waves combine their energies in constructive interfer-ence to produce powerful and intense laser light. Since the emitted light has consistentwavelength or colour, laser light is said to be monochromatic. Finally, the light wavesare emitted primarily in one direction.

These properties of coherence, intensity, and directionality make laser beams suitable for a wide range of scientific, commercial, medical, and military applications.Directionality ensures that a laser beam travels long distances in a straight line.

Figure 3A microwave communications toweruses parabolic reflectors to bothconcentrate the electromagneticradiation onto a small antenna whenreceiving a signal and to dispersethe radiation from a small antennawhen transmitting a signal.

incoherent light light of one ormore wavelengths, out of phase(e.g., white light)

coherent light light of one wave-length, in phase (e.g., laser light)

Assigning Frequency RangesThe Canadian and American gov-ernments, through the CanadianRadio-television andTelecommunications Commission(CRTC) and the United StatesFederal CommunicationsCommission (FCC), have the task ofassigning carrier frequency rangesfor various purposes, so that signalsdo not overlap and therefore pre-vent clear detection.

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Wave Effects of Light 537NEL

This property makes it useful for surveying, for establishing reference directions in thedrilling of tunnels, for measuring the very small movement in the continents, for deter-mining the speed of a baseball or a car, for guiding farmers in the laying of drainagetile, and for directing missiles. Further, since the light can be modulated, it is used infibre optics, permitting thousands of wires to be replaced with a single optical fibre.

Section 10.9

TRYTHIS activity Scattering Laser Light

Most of the divergence of a laser beam is caused by the scattering of light by air particles.To increase the scattering and make the beam more visible, clap two chalk dusterstogether above a laser beam in a dark room. Why can you now see the laser beam?

Do not let direct laser beams or reflected beams go straight intoanyone’s eyes.

If you are allergic or sensitive to chalk dust, use a light mist of waterinstead, from a spray bottle. Spray the water over the beam but keep themist away from the laser.

The intensity of laser beams makes them useful in cutting and welding materials,including metals. An industrial laser can easily concentrate ten thousand million wattsfor short intervals. With no blades to break or bits to wear, lasers can make precise cutsin fabric for suits, cauterize and cut in various types of surgery (including plastic surgery),weld dishwasher doors, plate easily corroded metals, etch the patterns of microcircuits,and drill eyes in surgical needles (Figure 6).

Figure 6Lasers have many applications, suchas cutting steel (left) and treatingskin conditions (right).

Figure 4This infrared photo reveals that thewindows are much warmer than thesurrounding structure. Replacingthe windows with double or triplethermopane windows will decreaseheat loss, conserving energy andreducing heating bills.

Figure 5Land-use patterns are clearly seenin this infrared image of the Albertaand Montana border area. Thicklyvegetated mountains and riverbanksappear red. Crops at different stagesof growth in Montana (bottom halfof image) appear in different colours.

Ultraviolet RadiationUltraviolet radiation (UV), also called ultraviolet light and sometimes “black light,” is aband of frequencies lying between visible light and X rays, with a frequency range from8.0 � 1014 Hz (violet light) to 1017 Hz. UV is emitted by very hot objects. Approximately7% of the Sun’s radiation is UV. It is this radiation that creates changes in the skin. Smallamounts of UV are necessary for the production of vitamin D in the human body. Butlarge amounts cause tanning and sunburn, which can increase the risk of skin cancerand cause cataracts. UV sources cause fluorescence in certain mineral ores and chemicalsubstances such as dyes and paints, which are used to dramatic effect in posters and T-shirts.

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Ionizing RadiationRadiation with frequencies higher than UV is ionizing. This radiation is so energeticthat it causes atoms in a substance to become ionized, expelling one or more electrons.Both X rays and gamma rays are ionizing. Radiation with frequencies of UV and loweris nonionizing. Ionization can break chemical bonds, that is, break up molecules. In thehuman body, ionizing radiation can cause cell death or possible alteration in the cellcausing illness and cancers. We will take a closer look at ionizing radiation in Unit 5.

ionizing radiation radiation at thelimit at which ionization can occur,at frequencies higher than ultraviolet

nonionizing radiation radiation ator below the limit at which ionizationcan occur, at frequencies lower thanultraviolet

Cell PhonesThe explosive worldwide growth in cell phone use hasincreased the public debate over possible health risks. Cellphones are small radio stations that send and receivemicrowaves. We noted in this section that radio waves, evenmicrowaves, are nonionizing radiations. Does it follow that theyare safe? Microwaves apparently do produce some heatingeffect on tissues, including the brain, near a cell phone. But it isnot clear whether this could have measurable biological effects,such as an increased risk for cancer.

Most studies have concluded “that the use of hand-held cel-lular telephones is not associated with the risk of brain cancer.”But the findings always have a caveat, such as “Further long-term studies are needed to account for longer usage, especiallyfor slow-growing tumours.”

Take a StandIs using a cell phone hazardous to your health?

Form an OpinionResearch the issue, citing the major studies done over the pastthree years. Make a note of the funding for each of the studiesyou examine.

• Evaluate the studies based on objective criteria.

• Summarize your findings and indicate to what extent the evi-dence objectively supports (or, as the case may be, is cur-rently insufficient to support) a conclusion.

• State your personal conclusion, defending your position.

• Will your research lead you to modify your cell phone habits?Why or why not?

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Define the Issue Analyze the Issue ResearchDefend the Position Identify Alternatives Evaluate

Decision-Making SkillsEXPLORE an issue

The Wave Theory of Light circa 1890At the end of the nineteenth century, the wave theory of light was firmly established.Double- and single-slit interference strongly corroborated the theory, while additionalinterference phenomena and polarization raised confidence levels still higher. With thework of Maxwell and Hertz, the physics community was left in little doubt that light, andwith it all other electromagnetic radiation, could be represented as transverse waves.Indeed, more radically, there was a sense that all the basic principles governing the phys-ical universe were now known, leaving physics nothing more than a responsibility toclean up details.

This complacency disappeared as the 1890s wore on. In Germany, Wilhelm Roentgendiscovered an unknown radiation, which he called “X rays,” that were powerful enoughto blacken a photographic plate after passing through flesh and wood. Shortly there-after, in France, Henri Becquerel accidentally left a piece of a uranium ore in a draweron top of a photographic plate. A few days later, he found that the plate was exposed bysome strange radiation and deduced that radiation from the uranium must have passedthrough the thick cover. And in 1897, the electron was firmly identified as the carrier ofelectricity by Joseph John (“J.J.”) Thomson in England.

However, soon thereafter, the complacent world of the scientists would be perma-nently disrupted. Radically new concepts of the physical world would emerge, ques-tioning the wave theory of electromagnetic radiation and Newtonian mechanics. Theseare the topics of the next unit.

Wave Effects of Light 539NEL

Section 10.9

• Radio waves originate from an oscillating electric field in an antenna and involvea carrier wave modulated by an audio and/or a video wave.

• Infrared radiation originates from a hot object that radiates progressively higher-frequency light as its temperature rises.

• Infrared radiation can be detected photographically and with infrared-sensitivecameras.

• Ultraviolet light has a high enough frequency that the rays can damage humantissue.

• The electromagnetic spectrum is further divided into two parts: ionizing andnonionizing radiation.

• By 1890, it was firmly established that light is an electromagnetic wave thattravels at 3.00 � 108 m/s in a vacuum.

Some Applications of Electromagnetic WavesSUMMARY

Section 10.9 QuestionsUnderstanding Concepts

1. Explain why medium-wave AM transmitters do not castshadows (regions of no reception) behind obstacles suchas buildings and hills, whereas TV and FM transmitters do.

2. An electromagnetic wave is travelling straight up, perpen-dicular to Earth’s surface. Its electric field is oscillating in aneast–west plane. What is the direction of oscillation of itsmagnetic field?

3. Explain why radio reception in a car is liable to distort whenthe car passes near high-voltage transmission lines orunder steel-reinforced concrete.

Applying Inquiry Skills

4. Some laser beams can be modulated. How would you setup a laser and a receiver so that you could transmit theaudio signal from a radio output across the room by laserlight? Try out your idea if you have suitable equipment.

Making Connections

5. Suntan lotions are rated 15 to 45. What is the relationshipbetween the rating and UV radiation?

6. Research the difference between UV-A and UV-B rays,labels commonly found on sunscreen lotion. Write a sum-mary of your findings.

7. Research medical applications of infrared radiation. Choosetwo uses, one in diagnosis and the other in treatment, andwrite a short report on each (maximum 500 words).

8. Give several examples, both favourable and adverse, ofways in which electromagnetic radiation directly influenceseither our health or our more general well-being.

9. Explain why cell-phone use is forbidden in hospitals.

10. Infrared and ultraviolet light have different heating effectson the human skin. Explain how these effects differ andwhy.

11. The heating effect of microwaves was originally noticed byradar operators at the end of World War II. They found thatthey could reheat meals by placing them near the mag-netron tubes generating the microwaves. This discoveryeventually led to the microwave oven. Research the Internetand other sources and answer the following questions:(a) What are the wavelengths of the microwaves used in

household microwave ovens?(b) What is their source?(c) How do microwaves heat food?(d) What types of food cannot be heated in a microwave

oven?(e) Draw a labelled diagram showing the construction of a

typical microwave oven. Comment on the safety fea-tures.

(f) Why was the practice of the radar operators dan-gerous?

Frequencies of TV Channelsand Radio StationsBroadcast frequencies range from500 kHz to 1600 kHz for medium-wave AM stations and from 88 MHz to 108 MHz for FM stations. TV carrier frequenciesrange from 54 MHz to 216 MHzfor VHF channels 2 to 13 and from470 MHz to 890 MHz for UHFchannels.

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