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Title: Chapter%208%20Electromagnetism%20and%20EM%20Waves%20(Section%205)


1
Chapter 8Electromagnetism and EM Waves(Section
5)
2
8.5 Electromagnetic Waves
  • Eyes, radios, televisions, radar, x-ray machines,
    microwave ovens, heat lamps. . . .
  • What do all of these things have in common?
  • They all use electromagnetic waves (EM waves).
  • EM waves occupy prominent places both in our
    daily lives and in our technology.
  • These waves are also involved in many natural
    processes and are essential to life itself.
  • In the rest of this chapter, we will discuss the
    nature and properties of electromagnetic waves
    and look at some of their important roles in
    todays world.

3
8.5 Electromagnetic Waves
  • As the name implies, EM waves involve both
    electricity and magnetism.
  • The existence of these waves was first suggested
    by 19th century physicist James Clerk Maxwell
    while he was analyzing the interactions between
    electricity and magnetism.

4
8.5 Electromagnetic Waves
  • Consider the two principles of electromagnetism
    stated in Section 8.3
  • Lets say that an oscillating electric field is
    produced at some place.
  • The electric field switches back and forth in
    direction while its strength varies accordingly.
  • This oscillating electric field will induce an
    oscillating magnetic field in the space around
    it.

5
8.5 Electromagnetic Waves
  • But the oscillating magnetic field will then
    induce an oscillating electric field.
  • This will then induce an oscillating magnetic
    field and so on in an endless loop
  • The principles of electromagnetism tell us that a
    continuous succession of oscillating magnetic and
    electric fields will be produced.

6
8.5 Electromagnetic Waves
  • These fields travel as a wave an EM wave.
  • Electromagnetic Wave A transverse wave
    consisting of a combination of oscillating
    electric and magnetic fields.
  • Electromagnetic waves are transverse waves
    because the oscillation of both of the fields is
    perpendicular to the direction the wave travels.

7
8.5 Electromagnetic Waves
  • The figure shows a snapshot of an EM wave
    traveling to the right.
  • The three axes are perpendicular to each other.
  • In this particular case, the electric field is
    vertical.

8
8.5 Electromagnetic Waves
  • As the wave travels by a given point in space,
    the electric field oscillates up and down, the
    way a floating petal oscillates on a water wave.
  • The magnetic field at the point oscillates
    horizontally.

9
8.5 Electromagnetic Waves
  • Electromagnetic waves do differ from mechanical
    waves in two important ways.
  • First, they are a combination of two waves in
    one an electric field wave and a magnetic field
    wave.
  • These cannot exist separately.
  • Second, EM waves do not require a medium in which
    to travel.
  • They can travel through a vacuum the light from
    the Sun does this.
  • They can also travel through matter.
  • Light through air and glass, and x-rays through
    your body are common examples.

10
8.5 Electromagnetic Waves
  • Electromagnetic waves travel at an extremely high
    speed.
  • Their speed in a vacuum, called the speed of
    light because it was first measured using light,
    is represented by the letter c.
  • Its value is
  • c 299,792,458 m/s (speed of light)
  • or
  • c 3108 m/s (approximately)
  • 300,000,000 m/s
  • 186,000 miles/s (approximately)

11
8.5 Electromagnetic Waves
  • All of the parameters introduced for waves in
    Chapter 6 apply to EM waves.
  • The wavelength can be readily identified in the
    figure.

12
8.5 Electromagnetic Waves
  • The amplitude is the maximum value of the
    electric field strength.
  • The equation v fl holds with v replaced by c.
  • There is an extremely wide range of wavelengths
    of EM waves, from the size of a single proton,
    about 1015 meters, to almost 4,000 kilometers
    for one type of radio wave.
  • The corresponding frequencies of these extremes
    are about 1023 hertz and 76 hertz, respectively.
  • Most EM waves used in practical applications have
    extremely high frequencies compared to sound.

13
8.5 Electromagnetic WavesExample 8.2
  • An FM radio station broadcasts an EM wave with a
    frequency of 100 megahertz.
  • What is the wavelength of the wave?
  • The prefix mega stands for 1 million. Therefore,
    the frequency is 100 million hertz.

14
8.5 Electromagnetic Waves
  • Electromagnetic waves are named and classified
    according to frequency.
  • In order of increasing frequency, the groups, or
    bands, are radio waves, microwaves, infrared
    radiation, visible light, ultraviolet radiation,
    x-rays, and gamma rays (G-rays).
  • Use of the word radiation instead of waves is not
    significant here.
  • This is called the electromagnetic spectrum.

15
8.5 Electromagnetic Waves
  • Notice that the groups overlap.
  • For example, a 1017-hertz EM wave could be
    ultraviolet radiation or an x-ray.
  • In cases of overlap, the name applied to an EM
    wave depends on how it is produced.

16
8.5 Electromagnetic Waves
  • We will briefly discuss the properties of each
    group of waves in the electromagnetic spectrum
  • how they are produced, what their uses are, and
    how they can affect us
  • The great diversity of uses of EM waves arises
    from the variety of ways in which they can
    interact with different kinds of matter.
  • All matter around us contains charged particles
    (electrons and protons), so it seems logical that
    EM waves can affect and be affected by matter.

17
8.5 Electromagnetic Waves
  • The oscillating electric field can cause AC
    currents in conductors
  • it can stimulate vibration of molecules, atoms,
    or individual electrons
  • or it can interact with the nuclei of atoms
  • Which sort of interaction occurs, if any, depends
    on the frequency (and wavelength) of the EM wave
    and on the properties of the matter through which
    it is traveling
  • its density, molecular and atomic structure, and
    so on

18
8.5 Electromagnetic Waves
  • In principle, an electromagnetic wave of any
    frequency could be produced by forcing one or
    more charged particles to oscillate at that
    frequency.
  • The oscillating field of the charges would
    initiate the EM wave.
  • The lower-frequency EM waves (radio waves and
    microwaves) are produced this way
  • A transmitter generates an AC signal and sends it
    to an antenna.

19
8.5 Electromagnetic Waves
  • At higher frequencies, this process becomes
    increasingly difficult.
  • Electromagnetic waves above the microwave band
    are produced by a variety of processes involving
    molecules, atoms, and nuclei.
  • Note that charged particles are present in all of
    these processes.

20
8.5 Electromagnetic Waves
  • There is one other factor to keep in mind
    electromagnetic waves are a form of energy.
  • Energy is needed to produce EM waves, and energy
    is gained by anything that absorbs EM waves.
  • The transfer of heat by way of radiation is one
    example.

21
8.5 Electromagnetic WavesRadio Waves
  • Radio waves, the lowest frequency EM waves,
    extend from less than 100 hertz to about 109 Hz
  • 1 billion hertz or 1,000 megahertz

22
8.5 Electromagnetic WavesRadio Waves
  • Within this range are a number of frequency bands
    that have been given separate names
  • for example, ELF (extremely low frequency), VHF
    (very high frequency), and UHF (ultrahigh
    frequency)
  • Most frequencies are given in kilohertz (kHz) or
    megahertz (MHz).
  • Sometimes radio waves are classified by
    wavelength
  • long wave, medium wave, or short wave

23
8.5 Electromagnetic WavesRadio Waves
  • As mentioned earlier, radio waves are produced
    using AC with the appropriate frequency.
  • Radio waves propagate well through the
    atmosphere, which makes them practical for
    communication.
  • Lower-frequency radio waves cannot penetrate the
    upper atmosphere, so higher frequencies are used
    for space and satellite communication.
  • Only the very lowest frequencies can penetrate
    ocean water.

24
8.5 Electromagnetic WavesRadio Waves
  • By far the main application of radio waves is in
    communication.
  • The process involves broadcasting a certain
    frequency of radio wave with sound, video, or
    other information encoded in the wave.
  • The radio wave is then picked up by a receiver,
    which recovers the information.

25
8.5 Electromagnetic WavesRadio Waves
  • Sometimes, this is a one-way process (commercial
    AM and FM radio and television), but in most
    other applications, it is two-way
  • Each party can broadcast as well as receive.
  • Narrow frequency bands are assigned for specific
    purposes.
  • For example, frequencies from 88 to 108 megahertz
    (88 million hertz to 108 million hertz) are
    reserved for commercial FM radio.
  • There are dozens of bands assigned to government
    and private communication.

26
8.5 Electromagnetic WavesMicrowaves
  • The next band of EM waves, with frequencies
    higher than those of radio waves, is the
    microwave band.
  • The frequencies extend from the upper limit of
    radio waves to the lower end of the infrared
    band, about 109 to 1012 hertz.
  • The wavelengths range from about 0.3 m to 0.3 mm.

27
8.5 Electromagnetic WavesMicrowaves
  • One use of microwaves is in communication.
  • Bluetooth and WiFi signals that interconnect
    computers, cell phones, and other devices are
    microwaves.
  • Early experiments with microwave communication
    led to the most important use of microwaves,
    radar (radio detection and ranging), after the
    discovery that microwaves are reflected by the
    metal in ships and aircraft.
  • Radar is echolocation using microwaves.

28
8.5 Electromagnetic WavesMicrowaves
  • The time it takes microwaves to make a round-trip
    from the transmitter to the reflecting object and
    back is used to determine the distance to the
    object.
  • Radar systems are quite sophisticated
  • Doppler radar can determine the speed of an
    object moving toward or away from the transmitter
    by measuring the frequency shift of the reflected
    wave.
  • Such radars are essential tools for air traffic
    control and monitoring severe weather.

29
8.5 Electromagnetic WavesMicrowaves
  • Since 2005, the Cassini spacecraft has used
    imaging radar to penetrate the dense, perpetual
    smog that envelopes Titan, Saturns largest moon,
    and to map its surface topology.
  • Similar radar equipment placed in orbit around
    Earth is used to form images of its surface, for
    such purposes as monitoring changes in the
    global environment and searching for geological
    formations (ancient craters, for example) and
    archaeological sites.

30
8.5 Electromagnetic WavesMicrowaves
  • Microwaves have gained wide acceptance as a way
    to cook food.
  • The goal of cooking is to heat the food, in other
    words, increase the energies of the molecules in
    the food.
  • Conventional ovens heat the air around the food
    and rely on conduction (in solids) or convection
    (in liquids) to transfer the heat throughout the
    food.

31
8.5 Electromagnetic WavesMicrowaves
  • Microwave ovens send microwaves (typically with f
    2,450 megahertz and l 0.122 meters) into the
    food.
  • The microwaves penetrate the food and raise the
    energies of the molecules directly.
  • Recall that water consists of polar
    moleculesthey have a net positive charge on one
    side and a net negative charge on the other side.

32
8.5 Electromagnetic WavesMicrowaves
  • The electric field of a microwave exerts forces
    on the two sides of the water molecules in food.
  • These forces are in opposite directions and twist
    the molecule.
  • Because the electric field is oscillating, the
    molecules are alternately twisted one way and
    then the other.

33
8.5 Electromagnetic WavesMicrowaves
  • This process increases the kinetic energy of the
    molecules and thereby raises the temperature of
    the food.
  • Cooking with microwaves is fast because energy is
    given directly to all of the molecules.
  • It does not rely completely on the conduction of
    heat from the outside to the inside of the food
    a much slower process.

34
8.5 Electromagnetic WavesInfrared
  • Infrared radiation (IR also called infrared
    light) occupies the region between microwaves and
    visible light in the electromagnetic spectrum.
  • The frequencies are from about 1012 hertz to
    about 41014 hertz (400,000,000 megahertz).
  • The wavelengths of IR range from approximately
    0.3 to 0.00075 millimeters.

35
8.5 Electromagnetic WavesInfrared
  • Infrared radiation is ordinarily the main
    component of heat radiation.
  • Everything around you is both absorbing and
    emitting infrared radiation, just as you are.
  • The warmth you feel from a fire or heat lamp is
    the result of your skin absorbing the IR.
  • Infrared radiation is constantly emitted by atoms
    and molecules because of their thermal vibration.
  • Absorption of IR by a cooler substance increases
    the vibration of the atoms and molecules, thus
    raising the temperature.

36
8.5 Electromagnetic WavesInfrared
  • Infrared radiation is commonly used in wireless
    remote-control units for televisions and for
    short-distance wireless data transfer between
    such devices as personal digital assistants
    (PDAs) and laptop computers.
  • These units emit coded IR that is detected by
    other devices.
  • In this capacity, IR is used much like radio
    waves.
  • Another use of IR is in lasers some of the most
    powerful ones in use emit infrared light.

37
8.5 Electromagnetic WavesVisible Light
  • Visible light is a very narrow band of
    frequencies of EM waves that happens to be
    detectable by human beings.
  • Certain specialized cells in the eye, called rods
    and cones, are sensitive to EM waves in this
    band.
  • They respond to visible light by transmitting
    electrical signals to the brain, where a mental
    image is formed.
  • The visible ranges of some animals such as
    hummingbirds and bees extend into the ultraviolet
    band.
  • Some flowers that seem plain to humans are quite
    attractive to these nectar eaters.

38
8.5 Electromagnetic WavesInfrared
  • Visible light is a component of the heat
    radiation emitted by very hot objects.
  • About 44 percent of the Suns radiation is
    visible light
  • it glows white hot
  • Incandescent light bulbs produce visible light in
    the same way.
  • Fluorescent and neon lights use excited atoms
    that emit visible light.

39
8.5 Electromagnetic WavesInfrared
  • Within the narrow band of visible light, the
    different frequencies are perceived by people as
    different colors.
  • The lowest frequencies of visible light, next to
    the infrared band, are perceived as the color
    red.
  • The highest frequencies are perceived as violet.

40
8.5 Electromagnetic WavesInfrared
  • The table shows the approximate frequencies and
    wavelengths of the six main colors in the
    rainbow.

41
8.5 Electromagnetic WavesInfrared
  • Note how narrow the frequency band is
  • The highest frequency of light we can see is less
    than twice the lowest.
  • By comparison, the range of frequencies of sound
    that can be heard is huge
  • The highest is 1,000 times the lowest.

42
8.5 Electromagnetic WavesInfrared
  • Most colors that you see are combinations of many
    different frequencies.
  • White represents the extreme
  • One way to produce white light is to combine
    equal amounts of all frequencies (colors) of
    light.

43
8.5 Electromagnetic WavesInfrared
  • Rainbow formation involves reversing the process
  • White light is separated into its component
    colors.
  • When no visible light reaches the eye, we
    perceive black.
  • In our daily lives, visible light is the most
    important of all electromagnetic waves.

44
8.5 Electromagnetic WavesUltraviolet Radiation
  • Ultraviolet (UV) radiation, also called
    ultraviolet light, is a band of EM waves that
    begins just above the frequency of violet light
    and extends to the x-ray band.
  • The frequency range is from about 7.51014 hertz
    to 1018 hertz.

45
8.5 Electromagnetic WavesUltraviolet Radiation
  • Ultraviolet light is also part of the heat
    radiation emitted by very hot objects.
  • About 7 percent of the radiation from the Sun is
    UV.
  • This part of sunlight is responsible for suntans
    and sunburns.
  • Ultraviolet radiation does not warm the skin as
    much as IR, but it does trigger a chemical
    process in the skin that results in tanning.
  • Overexposure leads to sunburn as a short-term
    effect, and repeated overexposure during a
    persons lifetime increases the chance of
    developing skin cancer.

46
8.5 Electromagnetic WavesUltraviolet Radiation
  • Some substances undergo fluorescence when
    irradiated with UV
  • They emit visible light.
  • The inner surfaces of fluorescent lights are
    coated with such a substance.
  • The UV emitted by excited atoms in the tube
    strikes the fluorescent coating, and visible
    light is produced.
  • The same process is used in plasma TVs.

47
8.5 Electromagnetic WavesUltraviolet Radiation
  • Some fluorescent materials appear to be colorless
    under normal light and can be used as a kind of
    invisible ink.
  • They can be seen under a UV lamp but are
    invisible otherwise.

48
8.5 Electromagnetic WavesUltraviolet Radiation
  • UV radiation has many practical applications.
  • For example, it is used as an investigative tool
    at crime scenes to help identify bodily fluids
    such as blood and bile.
  • Ultraviolet lights are used by entomologists to
    attract and collect nocturnal insects for
    cataloging and study.

49
8.5 Electromagnetic WavesUltraviolet Radiation
  • Ultraviolet lamps are used to sterilize
    workspaces and tools used in biology laboratories
    and medical facilities.
  • And, increasingly, ultraviolet lasers are finding
    use in many fields from metallurgy (engraving) to
    medicine (dermatology and optical keratectomy) to
    computing (optical data storage).

50
8.5 Electromagnetic WavesX-Rays
  • The next higher frequency electromagnetic waves
    are x-rays.
  • They extend from about 1016 to 1020 hertz.
  • An important feature of x-rays is that their
    range of wavelengths (about 108 to 1011 meters)
    includes the size of the spacing between atoms in
    solids.

51
8.5 Electromagnetic WavesX-Rays
  • X-rays are partially reflected by the regular
    array of atoms in a crystal and so can be used to
    determine the arrangement of the atoms.
  • X-rays also travel much greater distances through
    most types of matter compared to UV, visible
    light, and other lower-frequency EM waves.

52
8.5 Electromagnetic WavesX-Rays
  • X-rays are produced by smashing high-speed
    electrons into a target made of copper,
    tungsten or some other metal.
  • The electrons spontaneously emit x-rays as they
    are rapidly decelerated on entering the metal.
  • X-rays are also emitted by some of the atoms
    excited by the high-speed electrons.

53
8.5 Electromagnetic WavesX-Rays
  • Medical and dental x-ray photographs are made
    by sending x-rays through the body.
  • Typically, x-rays with frequencies between
    3.61018 hertz and 121018 hertz are used.
  • As x-rays pass through the body, the degree to
    which they are absorbed depends on the material
    through which they pass.

54
8.5 Electromagnetic WavesX-Rays
  • Tissue containing elements with relatively large
    atomic numbers (Z), such as calcium (Z 20),
    tend to absorb x-rays more effectively than those
    that contain predominantly light elements such as
    carbon (Z 6), oxygen (Z 8), or hydrogen (Z
    1).
  • Lead, with atomic number 82, is a particularly
    good shield for blocking x radiation.

55
8.5 Electromagnetic WavesX-Rays
  • Bones, which are rich in calcium, absorb x-rays
    better than soft tissue such as muscle or fat,
    and hence they show up more clearly on x-rays.

56
8.5 Electromagnetic WavesX-Rays
  • X-rays (and gamma rays) can be harmful because
    they are ionizing radiationradiation that
    produces ions as it passes through matter.
  • Such radiation can kick electrons out of atoms,
    leaving a trail of freed electrons and positive
    ions.
  • This process can break chemical bonds between
    atoms in molecules, thereby altering or
    destroying the molecule.

57
8.5 Electromagnetic WavesX-Rays
  • Living cells rely on very large, sophisticated
    molecules for their normal functioning and
    reproduction.
  • Disruption of such molecules by ionizing
    radiation can kill the cell or cause it to
    mutate, perhaps into a cancer cell.
  • The human body can (and does) routinely replace
    dead cells, but massive doses of x-rays or other
    ionizing radiation can overwhelm this process and
    cause illness, cancer, or death.

58
8.5 Electromagnetic WavesX-Rays
  • Because medical x-rays are the largest source of
    artificially produced radiation in the United
    States,
  • comprising about 10 percent of the total annual
    radiation dose for the average resident,
  • it is little wonder that protecting the public
    from unnecessary exposure to damaging radiation
    in diagnostic radiology is one of the greatest
    challenges to health and radiological physicists

59
8.5 Electromagnetic WavesGamma Rays
  • The highest-frequency EM waves are gamma rays
    (g-rays).
  • The frequency range is from about 31019 hertz to
    beyond 1023 hertz.
  • The wavelength of higher-frequency gamma rays is
    about the same distance as the diameter of
    individual nuclei.
  • Gamma rays are emitted in a number of nuclear
    processes
  • radioactive decay, nuclear fission, and nuclear
    fusion, to name a few

60
8.5 Electromagnetic WavesGamma Rays
  • This concludes our brief look at the
    electromagnetic spectrum.
  • Even though the various types of waves are
    produced in different ways and have diverse uses,
    the only real difference in the waves themselves
    is their frequency and, therefore, their
    wavelength.
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