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Vern J. Ostdiek
Donald J. Bord
Chapter 8
Electromagnetism and EM Waves
(Section 5)
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 today’s world.
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.
8.5 Electromagnetic Waves
• Consider the two principles of electromagnetism
stated in Section 8.3:
• Let’s 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.
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.
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.
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.
8.5 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.
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.
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 = 3×108 m/s (approximately)
= 300,000,000 m/s
= 186,000 miles/s (approximately)
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.
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 10–15 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.
8.5 Electromagnetic Waves
Example 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.
c= fl
300,000,000 m/s = 100,000,000 Hz ´ l
300,000,000 m/s
=l
100,000,000 Hz
l =3 m
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, xrays, and gamma rays (G-rays).
•
Use of the word radiation instead of waves is not
significant here.
• This is called the electromagnetic spectrum.
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.
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.
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
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.
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.
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.
8.5 Electromagnetic Waves
Radio 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
8.5 Electromagnetic Waves
Radio 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
8.5 Electromagnetic Waves
Radio 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.
8.5 Electromagnetic Waves
Radio 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.
8.5 Electromagnetic Waves
Radio 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.
8.5 Electromagnetic Waves
Microwaves
• 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.
8.5 Electromagnetic Waves
Microwaves
• 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.
8.5 Electromagnetic Waves
Microwaves
• 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.
8.5 Electromagnetic Waves
Microwaves
• Since 2005, the Cassini spacecraft has used
imaging radar to penetrate the dense, perpetual
smog that envelopes Titan, Saturn’s 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.
8.5 Electromagnetic Waves
Microwaves
• 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.
8.5 Electromagnetic Waves
Microwaves
• 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 molecules—
they have a net positive charge on one side and a
net negative charge on the other side.
8.5 Electromagnetic Waves
Microwaves
• 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.
8.5 Electromagnetic Waves
Microwaves
• 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.
8.5 Electromagnetic Waves
Infrared
• 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
4×1014 hertz (400,000,000 megahertz).
The wavelengths of IR range from approximately
0.3 to 0.00075 millimeters.
8.5 Electromagnetic Waves
Infrared
• 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.
8.5 Electromagnetic Waves
Infrared
• Infrared radiation is commonly used in wireless
remote-control units for televisions and for shortdistance 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.
8.5 Electromagnetic Waves
Visible 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.
8.5 Electromagnetic Waves
Infrared
• Visible light is a component of the heat radiation
emitted by very hot objects.
• About 44 percent of the Sun’s 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.
8.5 Electromagnetic Waves
Infrared
• 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.
8.5 Electromagnetic Waves
Infrared
• The table shows the approximate frequencies and
wavelengths of the six main colors in the rainbow.
8.5 Electromagnetic Waves
Infrared
• 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.
8.5 Electromagnetic Waves
Infrared
• 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.
8.5 Electromagnetic Waves
Infrared
• 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.
8.5 Electromagnetic Waves
Ultraviolet 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 xray band.
•
The frequency range is from about 7.5×1014 hertz
to 1018 hertz.
8.5 Electromagnetic Waves
Ultraviolet 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
person’s lifetime increases the chance of
developing skin cancer.
8.5 Electromagnetic Waves
Ultraviolet 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.
8.5 Electromagnetic Waves
Ultraviolet 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.
8.5 Electromagnetic Waves
Ultraviolet 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.
8.5 Electromagnetic Waves
Ultraviolet 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).
8.5 Electromagnetic Waves
X-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 10–8 to 10–11 meters) includes
the size of the spacing between atoms in solids.
8.5 Electromagnetic Waves
X-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.
8.5 Electromagnetic Waves
X-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.
8.5 Electromagnetic Waves
X-Rays
• Medical and dental “x-ray” photographs are made
by sending x-rays through the body.
•
Typically, x-rays with frequencies between
3.6×1018 hertz and 12×1018 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.
8.5 Electromagnetic Waves
X-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.
8.5 Electromagnetic Waves
X-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.
8.5 Electromagnetic Waves
X-Rays
• X-rays (and gamma rays) can be harmful because
they are ionizing radiation—radiation 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.
8.5 Electromagnetic Waves
X-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.
8.5 Electromagnetic Waves
X-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
8.5 Electromagnetic Waves
Gamma Rays
• The highest-frequency EM waves are gamma rays
(g-rays).
•
The frequency range is from about 3×1019 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
8.5 Electromagnetic Waves
Gamma 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.