34.7 The Spectrum of Electromagnetic Waves
The various types of electromagnetic waves are listed in Figure
34.13 (page 998), which shows the electromagnetic spectrum. Notice the wide ranges of frequencies and wavelengths.
No sharp
dividing point exists between one type of wave and the next. Remember that all
forms of the various types of radiation are produced by the same phenomenon:
acceleration of electric charges. The names given to the types of waves are
simply a convenient way to describe the region of the spectrum in which they
lie.
Radio
waves, whose wavelengths range from
more than 104 m
to about 0.1 m, are the result of charges accelerating through conducting
wires. They are generated by such electronic devices as LC oscillators
and are used in radio and television communication systems.
Microwaves
have wavelengths ranging from
approximately 0.3 m to 1024 m and are also generated by electronic devices. Because
of their short wavelengths, they are well suited for radar systems and for studying
the atomic and molecular properties of matter. Microwave ovens are an interesting
domestic application of these waves. It has been suggested that solar energy
could be harnessed by beaming microwaves to the Earth from a solar collector in
space.
Infrared
waves have wavelengths ranging from
approximately 1023 m to the longest wavelength of visible light, 7 3 1027
m. These waves, produced by
molecules and room-temperature objects, are readily absorbed by most materials.
The infrared (IR) energy absorbed by a substance appears as internal energy
because the energy agitates the object’s atoms, increasing their vibrational or translational
motion, which results in a temperature increase. Infrared radiation has
practical and scientific applications in many areas, including physical
therapy, IR photography, and vibrational spectroscopy.
Visible light, the most familiar form of electromagnetic waves,
is the part of the electromagnetic spectrum the human eye can detect. Light is
produced by the rearrangement of electrons in atoms and molecules. The various
wavelengths of visible light, which correspond to different colors, range from
red (l < 7 3 1027 m)
to violet (l < 4 3 1027 m). The sensitivity of the human eye is a
function of wavelength, being a maximum at a wavelength of about
5.5 3 1027 m. With
that in mind, why do you suppose tennis balls often have a yellow-green color?
Table 34.1 provides approximate correspondences between the wavelength of
visible light and the color assigned to it by humans. Light is the basis of the
science of optics and optical instruments, to be discussed in Chapters 35
through 38.
Ultraviolet
waves cover wavelengths ranging from
approximately 4 3 1027 m to 6 3 10210
m. The Sun is an important
source of ultraviolet (UV) light, which is the main cause of sunburn. Sunscreen
lotions are transparent to visible light but absorb most UV light. The higher a
sunscreen’s
solar protection factor, or SPF, the greater the percentage of UV light absorbed.
Ultraviolet rays have also been implicated in the formation of cataracts, a
clouding of the lens inside the eye.
Most of
the UV light from the Sun is absorbed by ozone (O3) molecules in the
Earth’s
upper atmosphere, in a layer called the stratosphere. This ozone shield
converts lethal high-energy UV radiation to IR radiation, which in turn warms
the stratosphere.
X-rays have wavelengths in the range from approximately 1028 m to 10212 m.
The most
common source of x-rays is the stopping of high-energy electrons upon bombarding
a metal target. X-rays are used as a diagnostic tool in medicine and as a
treatment for certain forms of cancer. Because x-rays can damage or destroy
living tissues and organisms, care must be taken to avoid unnecessary exposure or
overexposure.
X-rays
are also used in the study of crystal structure because x-ray wavelengths are
comparable to the atomic separation distances in solids (about 0.1 nm).
Gamma rays
are electromagnetic waves
emitted by radioactive nuclei and during certain nuclear reactions. High-energy
gamma rays are a component of cosmic rays that enter the Earth’s
atmosphere from space. They have wavelengths ranging from approximately 10210 m to
less than 10214 m. Gamma rays are highly penetrating and produce
serious damage when absorbed by living tissues. Consequently, those working
near such dangerous radiation must be protected with heavily absorbing materials
such as thick layers of lead.
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