Chapter 4Chapter 4Atmospheric OpticsAtmospheric Optics

Refraction is defined as the bending of raysas they pass through the atmosphere.

Refraction occurs whenever radiation travelsthrough 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.

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

Longer wavelength colors undergo less refraction than doshorter wavelength colors which concentrate near

the top of the Sun. Under some atmosphericconditions the Sun appears momentarily to be capped

by a bright green spot, known as the green flash.

Refraction of incoming solar radiation is greatest when theSun is low over the horizon, as the low solar angle causesthe rays to pass through a greater amount of atmosphere.

Mirages are caused by the refraction of visible light whenthe temperature decreases rapidly with increasing height.

The viewer at the left perceives distant objects to be slightly lower thanthey actually are. A person standing at position A appears slightly shorterthan he really is. At position B, the lower portion of his body appears tohave 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 bodydisappears from the bottom upward until he completely vanishes from sight.

If intensely heated air in the shallow layer just abovethe surface has an extremely steep temperature profile,

while the air immediately above the shallow layer is coolerand has a less steep vertical temperature profile, the

steeper temperature gradient of the lower layer causes itto refract air more strongly than does the air above it. As a result, a two-image inferior mirage can be seen.

Light is reflected off the treetop in all directions. Some of the light isdirected toward the viewer after undergoing only a small amount of

refraction producing the regular image of the tree. The reflected lightdirected toward the ground encounters the steep temperature gradient

causing strong refraction making the top of the tree appear belowthe ground and upside-down, producing a two-image inferior mirage.

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.

Rainbows are sweeping arcs of light that exhibit changesin color from the inner part of the ring to the outer part.

The brightest and most common are primary rainbowswith the shortest wavelengths of visible light appearing

at the innermost portion of the ring and thelonger 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.

Sunlight from behind the viewer undergoes reflection and refraction (a)to produce a primary rainbow. The amount of total refraction is differentfor each wavelength, causing the familiar color separation of a rainbow.

A viewer at ground level observes two concentric arcs creatinga primary and secondary rainbow (b).

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).

Cirrostratus clouds produce circular bands of light thatsurround the Sun or Moon, called halos that occur whenice crystals are between the viewer and the Sun or Moon.

Sundogs (or parhelia) appear as bright spots 22° to theright and left of the Sun due to platelike ice crystals larger

than 30 micrometers aligning themselves horizontally whenthe Sun is slightly above the horizon and behind these crystals.

Sundogs (or parhelia) appear as bright spots 22° to theright and left of the Sun due to platelike ice crystals larger

than 30 micrometers aligning themselves horizontally whenthe Sun is slightly above the horizon and behind these crystals.

Platelike crystals between a low Sun and an observer can alsoreflect sunlight off their tops and bottoms producing sun pillars.

Crystals are aligned almost horizontally with eachreflecting a portion of the incoming light differently to produce

apparent columns stretching up and down from the Sun.

Diffraction is the bending of light passing around water droplets.

The corona is a circular illumination of the skyimmediately surrounding the Moon or the Sun.

Larger droplets produce smaller coronas.

The glory is a series of rings around the shadow of an airplaneon the cloud deck as sunlight entering the edge of a waterdroplet is first refracted, then reflected off the inside of the

back of the droplet, and refracted again as it exits the droplet.

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.