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A Closer Look

Why Is the Sky Blue?
unlight entering our atmosphere is scat- Shorter-wavelength blue and violet light is When viewing a sunrise or sunset, you see
Stered, or redirected, by interactions with scattered more strongly than red and orange only light that has not been scattered in
air molecules. Sunlight appears to be white light. When you look at the sky, you see the other directions. The red and orange part of
to the human eye but is actually a mixture light that was redirected by the atmosphere sunlight travels through a maximum length
of all the colors of the rainbow. The blue into your line of sight. Since blue and vio- of the atmosphere, and the blue and violet
and violet part of the spectrum has shorter let light is scattered more efficiently than has been scattered away, so a sunrise or sun-
wavelengths than the red and orange part. red and orange light, the sky appears blue. set appears to be more orange and reddish.

light from the first crystal passes through the second with little
absorption. When the crystals are crossed at 90°, the light trans- Horizontally
mitted by the first is vibrating in a plane that is absorbed by the polarized

second crystal, and practically all the light is absorbed. At some light
other angle, only a fraction of the polarized light from the fi rst Unpolarized
crystal is transmitted by the second. light
You can verify whether a pair of sunglasses is made of
polarizing material by rotating a lens of one pair over a lens of a
second pair. Light is transmitted when the lenses are aligned but
mostly absorbed at 90° when the lenses are crossed.
FIGURE 7.22 Light that is reflected becomes partially or fully
Light is completely polarized when all the waves are
polarized in a horizontal direction, depending on the incident angle
removed except those vibrating in a single direction. Light is and other variables.
partially polarized when some of the waves are in a particular
orientation, and any amount of polarization is possible. Th ere
are several means of producing partially or completely polar- molecules that make up the air. Sunlight is initially unpolar-
ized light, including (1) selective absorption, (2) refl ection, and ized. When it strikes a molecule, electrons are accelerated
(3) scattering. and vibrate horizontally and vertically. The vibrating charges
Selective absorption is the process that takes place in certain reradiate polarized light. Thus, if you look at the blue sky
crystals, such as tourmaline, where light in one plane is trans- with a pair of polarizing sunglasses and rotate them, you will
mitted and light in all the other planes is absorbed. A method of observe that light from the sky is polarized. Bees are believed

manufacturing a polarizing film was developed in the 1930s by to be able to detect polarized skylight and use it to orient
Edwin H. Land (1909–1991). Th e film is called Polaroid. Today, the direction of their flights. Violet light and blue light have

Polaroid is made of long chains of hydrocarbon molecules that the shortest wavelengths of visible light, and red light and
are aligned in a fi lm. The long-chain molecules ideally absorb orange light have the largest. The violet and blue rays of sun-

all light waves that are parallel to their lengths and transmit light are scattered the most. At sunset the path of sunlight
light that is perpendicular to their lengths. The direction that through the atmosphere is much longer than when the Sun

is perpendicular to the oriented molecular chains is thus called is more directly overhead. Much of the blue and violet has
the polarization direction or the transmission axis. been scattered away as a result of the longer path through
Refl ected light with an angle of incidence between 1° and the atmosphere at sunset. The remaining light that comes
89° is partially polarized as the waves parallel to the refl ecting through is mostly red and orange, so these are the colors you
surface are reflected more than other waves. Complete polariza- see at sunset.

tion, with all waves parallel to the surface, occurs at a particular

angle of incidence. This angle depends on a number of variables, 7.4 EVIDENCE FOR PARTICLES

including the nature of the reflecting material. Figure 7.22 illus-
trates polarization by reflection. Polarizing sunglasses reduce The evidence from diffraction, interference, and polarization of

the glare of reflected light because they have vertically ori- light was very important in the acceptance of the wave theory

ented transmission axes. This absorbs the horizontally oriented because there was simply no way to explain these behaviors
reflected light. If you turn your head from side to side so as to with a particle theory. Then, in 1850, J. L. Foucault (pronounced

rotate your sunglasses while looking at a reflected glare, you will “Foo-co”) (1819–1868) was able to prove that light travels much

see the intensity of the reflected light change. This means that more slowly in transparent materials than it does in air. Th is
the reflected light is partially polarized. was in complete agreement with the wave theory and com-

The phenomenon called scattering occurs when light is pletely opposed to the particle theory. By the end of the 1800s,
absorbed and reradiated by particles about the size of gas James Maxwell’s (1831–1879) theoretical concept of electric and
192 CHAPTER 7 Light 7-16