1. Chapter 4
Atmospheric Optics

2. Refraction is defined as the bending of rays
as they pass through the atmosphere.
Refraction occurs whenever radiation travels
through a medium whose density varies or whenever it
passes from one medium to another having a different density.
The amount and direction of refraction
vary with atmospheric conditions.

3. The light from the tall building is bent downward, so its path is
concave downward. The light reaches the viewer’s eye at
an angle slightly greater than it would without refraction,
making the top of the building appear higher than it is.

4. Refraction of incoming solar radiation is greatest when the
Sun is low over the horizon, as the low solar angle causes
the rays to pass through a greater amount of atmosphere.
Longer wavelength colors undergo less refraction than do
shorter wavelength colors which concentrate near
the top of the Sun. Under some atmospheric
conditions the Sun appears momentarily to be capped
by a bright green spot, known as the green flash.

5. Mirages are caused by the refraction of visible light when
the temperature decreases rapidly with increasing height.
The viewer at the left perceives distant objects to be slightly lower than
they actually are. A person standing at position A appears slightly shorter
than he really is. At position B, the lower portion of his body appears to
have disappeared because the light reflected off his legs is bent all the
way to the ground. Moving toward position C, more and more of his body
disappears from the bottom upward until he completely vanishes from sight.

6. If intensely heated air in the shallow layer just above
the surface has an extremely steep temperature profile,
while the air immediately above the shallow layer is cooler
and has a less steep vertical temperature profile, the
steeper temperature gradient of the lower layer causes it
to refract air more strongly than does the air above it.
As a result, a two-image inferior mirage can be seen.

7. directed toward the viewer after undergoing only a small amount of
Light is reflected off the treetop in all directions. Some of the light is
directed toward the viewer after undergoing only a small amount of
refraction producing the regular image of the tree. The reflected light
directed toward the ground encounters the steep temperature gradient
causing strong refraction making the top of the tree appear below
the ground and upside-down, producing a two-image inferior mirage.

8. A superior mirage forms when images are displaced upward.
Light rays are bent concave downward as a result of
decreasing density with increasing height.
The normal density gradient must be enhanced by a
temperature profile in which warm air lies above cold air.
There may be stretching or compression of the image,
depending on how refraction varies with altitude.

9. Rainbows are sweeping arcs of light that exhibit changes
in color from the inner part of the ring to the outer part.
The brightest and most common are primary rainbows
with the shortest wavelengths of visible light appearing
at the innermost portion of the ring and the
longer wavelengths framing the outermost portion.
The less distinct secondary rainbow covers about
100 degrees of arc at the horizon and has
the reverse color scheme of the primary rainbow.

10. A viewer at ground level observes two concentric arcs creating
Sunlight from behind the viewer undergoes reflection and refraction (a)
to produce a primary rainbow. The amount of total refraction is different
for each wavelength, causing the familiar color separation of a rainbow.
A viewer at ground level observes two concentric arcs creating
a primary and secondary rainbow (b).

11. Cirrostratus clouds produce circular bands of light that
surround the Sun or Moon, called halos that occur when
ice crystals are between the viewer and the Sun or Moon.
Column-shaped and platelike crystals refract light to produce a 22o halo (a).
Refraction where ice crystals have 90o angles produces a 46o halo (b).

12. Sundogs (or parhelia) appear as bright spots 22° to the
right and left of the Sun due to platelike ice crystals larger
than 30 micrometers aligning themselves horizontally when
the Sun is slightly above the horizon and behind these crystals.

13. Sundogs (or parhelia) appear as bright spots 22° to the
right and left of the Sun due to platelike ice crystals larger
than 30 micrometers aligning themselves horizontally when
the Sun is slightly above the horizon and behind these crystals.

14. Platelike crystals between a low Sun and an observer can also
reflect sunlight off their tops and bottoms producing sun pillars.
Crystals are aligned almost horizontally with each
reflecting a portion of the incoming light differently to produce
apparent columns stretching up and down from the Sun.

15. Diffraction is the bending of light passing around water droplets.
The corona is a circular illumination of the sky
immediately surrounding the Moon or the Sun.
Larger droplets produce smaller coronas.
The glory is a series of rings around the shadow of an airplane
on the cloud deck as sunlight entering the edge of a water
droplet is first refracted, then reflected off the inside of the
back of the droplet, and refracted again as it exits the droplet.

17. Glories require diffraction along the edge of a cloud droplet as the
sunlight exits the droplet. The bending from refraction returns the
sunlight almost 180o from the direction at which it entered the droplet.