Atmospheric Optics - II
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RECAP
Human perception of color, white objects, black objects.
Light scattering: light is sent in all directions –forward, sideways and
backward
♦ Geometric scattering: R>>l (all wavelengths equally scattered)
♦ Mie scattering: R~l (red is scattered more efficiently)
♦ Rayleigh scattering: R<<l (blue is scattered more efficiently)
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• Refraction: the bending of the light ray as it travels from one medium to
Phenomena: white clouds, blue skies, hazy skies, crepuscular rays, colorful
sunsets, blue moon.
TODAY:
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another. It bends towards the vertical if it enters a more-dense medium
and away from the vertical as it enters a less-dense medium.
♦ Phenomena: stars appear higher in the sky, twinkling, twilight.
Reflection: the angle of incidence is equal to the angle of reflection
Total internal reflection: mirages
Dispersion: separation of colors when light travels through a medium.
♦ rainbow
Reflection and Refraction of Light
• The
speed of light in vacuum
is c=300,000 km/s
• Snell’s
law: The angle of
incidence is equal to the
angle of reflection.
• Light
that enters a moredense medium slows down and
bends toward the normal.
• Light
that exits a moredense medium speeds up and
bends away from the normal.
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True and apparent position of objects
Due to the refraction of
light the objects on the
sky appear higher than
they actually are.
♦ Star location and
scintillations;
♦ Timing of the sunset
and the sunrise;
♦ The sun on the horizon
looks flattened;
♦ Twilight.
The Timing of the Sunset & Sunrise
We see the sun before it actually rises above the horizon and after it
sets below the horizon.
Twilight
Flattening of the Sun’s Disk at Sunset
Green flash
Flattening of the Moon
Refraction by the Earth’s atmosphere (image from ISS)
http://www.sundog.clara.co.uk/atoptics/phenom.htm
Total Internal Reflection
•The light travels from more dense towards less dense medium.
•If the incident angle exceeds the critical angle the light is fully reflected.
Refraction
(less dense->dense)
Refraction
(dense->less dense)
water
q1
air
q1
Total internal reflection
(dense->less dense)
qcr
water
air
water
q2
Cold air
q2
air
Hot air
q1>qcr
Mirages
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• refracted due to changes of the atmospheric density.
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Description: an object is displaced from its true
position.
Physical process: the light form the object is strongly
Classification:
♦ Inferior: pool on the road, oasis in the desert
♦ Superior: mountains and castles on the sea horizon
Thin layer of hot, less-dense air
• Conditions for an inferior (inverted) mirage
• Conditions for a superior (upright) mirage
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Fata Morgana (a.k.a.Morgan le Fay)
Objects on the horizon, such as islands, cliffs, ships or
icebergs, appear elongated and elevated, like "fairy tale
castles".
It is frequently seen where warm air rests over a cold
surface, such as above large bodies of water and in polar
regions
A highway mirage
A desert mirage
A Fata Morgana seen on the Norwegian coast
Sunset mirage
Superior mirage: a layer of abnormally warm air above cooler air.
http://www.sundog.clara.co.uk/atoptics/phenom.htm
Etruscan Vase Sunset Mirage
• Astwotheeventually
sun sinks a second sun appears to rise from the water. The
join at a thin red colored stem.
• The
lower sun is not a reflection from the water. It is an inferior
mirage. The lower sun, an inverted image of the upper one, is produced
by refraction in a lower layer of warm and less dense air.
http://www.sundog.clara.co.uk/atoptics/phenom.htm
Sunset mirage (pancake suns)
Here the atmosphere's vertical temperature profile is complex. There
are at least three layers with different temperatures and the central
layer is possibly cooler than those above and below it. Each pancake is an
image of a particular slice through the sun.
http://www.sundog.clara.co.uk/atoptics/phenom.htm
Optical Effects from Ice Crystals
22 deg Sun Halo
• It is caused by randomly oriented ice crystals with a
shape of hexagonal prism.
• The crystals whose axes are roughly perpendicular to the
direction of sunlight refract its rays through 22º or more.
Each sparkles in the sky when it is at this angular distance
from the sun. The collective sparkles form the halo.
Sun Dogs
Dispersion
Tangent Arcs
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The long axes of
the ice crystals
are horizontal to
the ground.
Sun pillars
• horizontally oriented and wobble as they fall
Reflection from plate-like ice crystals that are
Optical effects from water droplets
Rainbow
Primary
Bow
Secondary
Bow
Bright sky
inside the bow
Alexander’s
dark band
Rainbows are caused by internal reflection of solar light by raindrops.
Rainbow Viewing Geometry
•The observer is between the sun and the rainbow.
•The rainbow angle is 42.5 degrees.
Creating a Rainbow
• a single internal reflection from
The primary rainbow is caused by
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the raindrop.
The secondary rainbow is the
result of a double internal
reflection from the surface of
the raindrop.
The color separation is due to
dispersion of light in the process
of refraction.
The red color is on the outside of
the primary and on the inside of
the secondary bow.
in
out
Summary
• Mirages are caused by atmospheric layers with different densities:
♦ Inferior (e.g. wet road): cold air over hot air
♦ Superior (e.g. Fata Morgana): hot air over cold air
• Water droplets cause:
♦ Rainbows
Primary
Secondary (2 internal reflections)
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• Ice crystals cause:
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Sundogs
Halos
Tangents
Sun pillars