The chapter opened with a discussion
of angle measurement in the sky, followed by a short review of
the particle and wave models of light and the relation between
color and wavelength of light, then introduction of the terms
ray and beam. We saw the role of scattering in creating
the shafts of light in crepuscular rays and observed the occurrence
of shadows in the atmosphere in other phenomena such as the Spectre
of the Brocken and earth shadow. Variations in the atmosphere's density
affect the apparent positions of the sun, moon, and other objects,
causing them to appear slightly higher in the sky than they actually
are. Sometimes the position changes are more extreme, resulting
in mirages. Density variations also are responsible for alterations
in the shapes of some objects and the twinkling of stars. All
of these effects are the result of the refraction of light. Refraction
is a particularly important player in atmospheric optics. Figure
15.34 completes the concept map begun earlier (Figure 15.13) on
the relation of refraction to the phenomena discussed in this
chapter. Sky color—blue by day, other colors
near sunrise and sunset—is caused by Rayleigh scattering of light
by air molecules and other extremely small particles in the atmosphere.
Rayleigh scattering is far more effective for light of short wavelengths—blue
and violet in the visible spectrum. Cloud particles, being much
larger than air molecules, scatter light of all wavelengths equally,
causing them to appear white. Dark clouds are simply clouds in
shadow. Refraction of sunlight or moonlight
by tumbling columnar ice crystals can cause halos around the moon
or sun, or other effects such as sun dogs. Rings of light close
to the moon or sun, known as coronas, are products of diffraction,
the bending of light around objects (cloud droplets, in the case
of coronas). Rainbows are the result of refraction
and reflection of sunlight that passes into and out of falling
raindrops. Rainbows, unlike halos and coronas, appear on the opposite
side of the sky from the sun. Often a faint secondary bow is visible
outside the primary bow, and sometimes supernumerary bows appear
within the primary bow. The beautiful set of rings called
the glory, visible below airborne or mountaintop observers, is
caused by refraction, reflection, and diffraction within water
droplets.
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