gjphw@ihuxm.UUCP (02/23/84)
The reason why the sky appears blue should be attributed to the confluence
(yes, I looked this word up) of three properties.
First, the atmosphere scatters light preferentially. Violet light is scat-
tered more effectively than red light (inverse fourth power of the
wavelength). The classical reason is called Rayleigh scattering (scattering
off of air molecules and very fine dust), but I am partial to the density
fluctuations in the atmosphere explanation. These fluctuations cause fluctua-
tions in the local index of refraction, which integrate to an inverse fourth
power of the wavelength scattering behavior. With this scattering alone, and
a uniform (white) spectrum from the sun, the sky should appear violet, not
blue.
Second, the sun's light output is much reduced in the violet compared with
the yellow-green. In fact, the sun's black body color is slightly yellow of
green. I have read that the yellow is primarily due to calcium in the sun.
Third, our eyes are most sensitive in the green (all the better to distin-
guish among the foliage). The combination of these three properties results
in our perception of the sky as blue. The astronauts have claimed that film
and artists' renditions do not adequately represent the colors that they can
see in the atmosphere while in orbit. Part of their perceptions probably
derives from the spectrum of reflected light from the Earth's surface.
Sunsets are redder than midday because the light must travel a much longer
path through the atmosphere, making the scattering more effective. Add some
additional fine dust provided by a volcano or industrial area, and the sunsets
become even redder, though also darker.
With this information, you can now answer the following two questions:
Which are redder, sunrises or sunsets?
Since the sky is blue, what is Avogadro's number (a prelim question)?
--
Patrick Wyant
AT&T Bell Laboratories (Naperville, IL)
*!ihuxm!gjphw