Physics For Scientists And Engineers 6E - part 271
light, which correspond to different colors, range from red (*
$ 7 $ 10
)
7
m) to violet
(*
$ 4 $ 10
)
7
m). The sensitivity of the human eye is a function of wavelength, being
a maximum at a wavelength of about 5.5 $ 10
)
7
m. With this in mind, why do you
suppose tennis balls often have a yellow-green color?
Ultraviolet waves cover wavelengths ranging from approximately 4 $ 10
)
7
m to
6 $ 10
)
10
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 (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 (O
3
) molecules in the
Earth’s upper atmosphere, in a layer called the stratosphere. This ozone shield
converts lethal high-energy UV radiation to infrared radiation, which in turn warms
the stratosphere. Recently, a great deal of controversy has arisen concerning the possi-
ble depletion of the protective ozone layer as a result of the chemicals emitted from
aerosol spray cans and used as refrigerants.
X-rays have wavelengths in the range from approximately 10
)
8
m to 10
)
12
m. The
most common source of x-rays is the stopping of high-energy electrons upon bombard-
ing 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 damage or destroy living tissues and organ-
isms, 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 (such as
60
Co and
137
Cs) and during certain nuclear reactions. High-energy gamma rays are a
S E C T I O N 3 4 . 6 • The Spectrum of Electromagnetic Waves
1081
Wavelength
1 pm
1 nm
1
µ
m
1 cm
1 m
1 km
Long wave
AM
TV, FM
Microwaves
Infrared
Visible light
Ultraviolet
X-rays
Gamma rays
Frequency, Hz
10
22
10
21
10
20
10
19
10
18
10
17
10
16
10
15
10
14
10
13
10
12
10
11
10
10
10
9
10
8
10
7
10
6
10
5
10
4
10
3
µ
Radio waves
1 mm
Violet
Blue
Green
Yellow
Orange
Red
~400 nm
~700 nm
Figure 34.12 The electromagnetic spectrum. Note the overlap between adjacent wave
types. The expanded view to the right shows details of the visible spectrum.
Wearing sunglasses that do not
block ultraviolet (UV) light is worse
for your eyes than wearing no
sunglasses. The lenses of any
sunglasses absorb some visible
light, thus causing the wearer’s
pupils to dilate. If the glasses do
not also block UV light, then more
damage may be done to the lens of
the eye because of the dilated
pupils. If you wear no sunglasses at
all, your pupils are contracted, you
squint, and much less UV light
enters your eyes. High-quality
sunglasses block nearly all the
eye-damaging UV light.
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