Light, Color,

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Light, Color, Atmospheric Optics

• This chapter discusses
• Various optical phenomena (e.g. blue skies,
  rainbows, mirages, and coronas)
• Reflection, scattering, refraction, diffraction

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Reflected Light
Figure 4.1
Human eyes are sensitive to light color, and we
see objects because of the light and color that
they reflect.
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White Dark Clouds
Cloud water droplets are poor absorbers of light,
but large enough to reflect all wavelengths as
geometric scatterers. By scattering all visible
wavelengths the cloud creates a white passage of
light. Thick clouds, however, diminish the
passage of light, and appear dark.
Figure 4.2
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Reflected, Transmitted, Absorbed
Figure 4.3
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Solar energy in the 0.4 to 0.7 micron range is
visible and conserved (e.g. not created or
destroyed). As a wave of light passes through a
cloud, only a small amount is absorbed, and the
rest is either reflected or transmitted as a
function of cloud thickness.
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Scattering Blue Skies
Figure 4.4
Air molecules are only large enough to reflect
the smallest wavelength light, which is blue.
Rayleigh scattering takes a beam of sunlight and
selectively creates blue skies. A yellow sun is
evidence of this removed blue light.
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Blue Skies White Clouds
Figure 4.5
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Blue Haze Selective Scatter
Figure 4.6
Plants exude terpenes that react with ozone and
can also selectively scatter blue wavelengths,
creating a blue haze. Mountains allow for enough
viewing distance to create this needed scatter.
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Atmospheric Dust Rays
Figure 4.7
Dust and salts are large enough to cause
geometric scattering, and change blue skies into
hazy white skies. Concentrated dust or salts
beneath clouds can create white crepuscular rays
of sunlight.
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Colorful Sun Clouds
Figure 4.8
Low on the horizon, the sun's light passes
through nearly 12 times more atmospheric gas and
aerosol, which scatters most short wavelengths.
Longer oranges and reds then comprise the
sunlight, which reflects from clouds and water.
Figure 4.9
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Light Refraction Position
Figure 4.10
Light entering earth's atmosphere is slowed by
the increased gas density, and bends toward the
normal. Such refraction explains why light bends
when entering water from air, stars are actually
lower on the horizon, and stars may flicker.
Figure 4.11
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Refraction Twilight
Figure 4.12
Refraction, or bending, of sunlight creates an
apparent early sunrise and late sunset. The
amount of extra daylight increases with latitude
during Northern summers.
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Colors of Twilight
Figure 4.14
Figure 4.13
Twilight may become more colorful from added
atmospheric pollutants, such as sulfurs from
volcanic eruptions, or from green flashes of
light caused by scattering shorter wavelengths.
Refraction will also cause the sun to flatten at
its base.
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Inferior Mirage
Figure 4.16
Refraction of light by density differences is the
cause for a mirage, creating an apparent
figure. When very hot air rests below cooler
air, light transmitted light bends upwards,
creating a line of sight into the ground.
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Superior Mirage
Figure 4.17
A superior mirage occurs when warm air rests
above cold air, causing the line of sight to head
upward and the object appear at a higher altitude.
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Ice Crystal Halo
Figure 4.18
Figure 4.19
Cirriform cloud ice crystals randomly oriented to
the ground refract light at an angle of 22 to
create an arc. Less common are the 46 halo,
which require more regular column-type crystals.
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Halo Tangent Arc
An arc of light forms tangent to the halo when
large hexagonal crystals fall with their long
axis horizontal to the ground.
Figure 4.20
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Dispersion of Light
A glass prism slows light, and the refraction
selectively bends shorter blue light more than
longer red light. Ice crystals can create a
similar dispersion of light.
Figure 4.21
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Sun Dogs Dispersion
Figure 4.22
Ice crystals selectively refract and bend
sunlight to create brightly colored red spots on
either side of the sun, called sun dogs or
parahelia.
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Sun Dog in Glacier Bay, Alaska
Figure 4.23
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Reflection Sun Pillar
Sun pillars are formed by reflection of light
off, not refraction of light through, ice
crystals falling in still air.
Figure 4.24
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Summary of Ice Crystal Phenomena
Figure 4.25
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Rainbows Rainfall
Rainbows, and red skies, have been used to
foretell the weather. Rainbows are only seen
when the viewer has their back to the sun and
raindrops before them. Because weather is
predominantly from the west, a rainbow at sunset
means the storm has already passed.
Figure 4.26
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Rainbow Reflection Refraction
Sunlight must enter the raindrop at greater than
the critical angle of 48 to be internally
reflected. As the light enters/leaves the drop,
it refracts and disperses into separate colors,
red bending least.
Figure 4.27A
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Primary Rainbow
Red bends least, and leaves the raindrop below
the blue and violet colors. An observer,
however, sees a rainbow by capturing one color
per drop, where red and its small bend is now
required to reside at the rainbow top.
Figure 4.28
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Double Reflection for Secondary Bow
Figure 4.29
Figure 4.30
Sunlight entering a raindrop may reflect twice
before exiting, which reverses the color sequence
and diminishes the rainbow intensity.
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Diffraction Coronas
Figure 4.32
Figure 4.31
Solar and lunar coronas are caused by
diffraction, or bending of light as it passes
around (not through) ice crystals or
pollutants. Diffraction causes circles of
increased illumination where a wave of light
reinforces other waves of light.
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Diffraction Colors
Figure 4.33
Bending white light by non-uniform cloud droplets
causes distorted separation of the different
wavelengths, as seen in this colorful cloud based
corona.
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Glory Surface Waves
Figure 4.34
Figure 4.35
The glory around the aircraft shadow is
attributed to diffraction, but based partly on
surface refraction to create the bending that
brings the light back to the viewer.