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Lecture 17! !
Nature’s
Light Show
Atmospheric OpticsScatteringReflection
Ahrens Chapter 15
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Atmospheric Optics
Nature’s Light Show
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Atmospheric Optics
• Scattering• Reflection• Refraction • Diffraction
The amazing variety of optical phenomena observed in the atmosphere can be explained by four physical mechanisms.
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Scattering
Things to look for• Blue Sky• White Clouds• Blue Smoke• Red Sunsets• Crepuscular Rays• Heiligenschein
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Scattering
Light is scattered by the air molecules, cloud droplets, and aerosols.
The resulting optics depend on the size of the scatterer.
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Scattering
Rayleigh Scattering Small objects such
as air molecules and fine smoke particles most effectively scatter blue light.
sunlight
air molecule
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Rayleigh Scattering
Scattering of blue light by air molecules is more than 9 times greater than scattering of red light.
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Scattering by Air Molecules
Rayleigh Scattering results in blue sky
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Scattering by Air Molecules
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Scattering
Mie Scattering Larger objects such as cloud droplets and ice crystals scatter all visible light equally well.
sunlight
Cloud drop
Mie scattering is greatest parallel to incident light.
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Scattering
Mie scattering is greatest parallel to incident light. Rayleigh scatter is nearly equal in all direction.
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Scattering by Air Molecules and Clouds
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Scattering by Air Molecules and Aerosols
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Rayleigh and Mie Scattering
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Rayleigh and Mie Scattering
Mie scattering results in white clouds and the glare around the sun. Blue sky is the result of Rayleigh scattering.
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Scattering by Smoke
Larger particles appear brown and smaller particles scatter blue.
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Scattering by Cloud Droplets
Mie scattering results in white clouds, with black bottoms if they are thick enough.
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When all visible light is scattered away by cloud drops the cloud turns black.
Scattering by Cloud Particles
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Avoid exposed
places during
thunderstorms
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Most of the blue light is scattered out, leaving red light, which is scattered toward the observer by clouds.
Red Sunsets
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Scattering by Cloud Particles
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When the sun sets or rises, the sunlight passes through a long path of air. Most of the blue light is scattered out, leaving red light, which is scattered toward the observer by clouds.
Scattering by Cloud Particles
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Scattering by Aerosols
Crepuscular Rays23
Crepuscular Rays
Scattering by Aerosols
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Crepuscular Rays
Scattering by Aerosols
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Scattering by Cloud Droplets
Crepuscular Rays
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Crepuscular Rays
Shadows cast by clouds or trees on hazy days result in crepuscular rays, also known as Jacob’s ladders.
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Crepuscular Rays
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Crepuscular Rays
Mountain’s Shadow
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Anti-crepuscular Rays
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Reflection
The Law of Reflection The angle a) of incident light equals the angle b)
of reflected light.
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Reflection
• Sun Pillars• Circumhorizontal Arcs
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Reflection by Water
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Reflection by Water
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Reflection by Water
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Reflection by Water
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Sunlight reflecting off of the ocean can produce a sun pillar.
Reflection by Water
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Reflection by Ice Crystals
Sun Pillars
The Law of Reflection The angle a) of incident light equals
the angle b) of reflected light.
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Sunlight reflecting off of plate-shaped ice crystals can produce a sun pillar.
Reflection by Ice Crystals
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Sun pillar
Reflection by Ice Crystals
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Sun pillars commonly occur beneath an altostratus cloud just after sunset.
Reflection by Ice Crystals
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Sun pillars commonly occur beneath an altostratus cloud just after sunset.
Reflection by Ice Crystals
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Sun pillars commonly occur beneath an altostratus cloud just after sunset.
Reflection by Ice Crystals
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Reflection and Scattering
by Dew Drops
Heiligenschein
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Reflection and Scattering
by Dew Drops
Heiligenschein
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Questions?
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Atmospheric Optics
• Scattering• Reflection• Refraction • Diffraction
The amazing variety of optical phenomena observed in the atmosphere can be explained by four physical mechanisms.
47
Rayleigh and Mie Scattering
Things to look for• Blue Sky• White Clouds• Blue Smoke• Red Sunsets• Crepuscular Rays• Heiligenschein
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When the sun sets or rises, the sunlight passes through a long path of air. Most of the blue light is Rayleigh scattered out, leaving red light, which is Mie scattered toward the observer by clouds.
Scattering by Cloud Particles
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Scattering by Cloud Particles
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Reflection
• Sun Pillars• Circumhorizontal Arcs
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Refraction
• Green Flash• Mirage• Halo• Tangent Arc• Rainbow
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Refraction
Light slows down as it passes from a less dense to a more dense medium.As light slows it bends toward the denser medium. Similar to waves approaching a beach.The amount of bending depends on the wavelength (color) of the light, leading to dispersion or separation of colors.
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Refraction in Air
Mirage
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Refraction in Air
Mirage
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Refraction in Air
Green Flash
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Refraction in Air
Green Flash
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Refraction in Air
Green Flash
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Refraction in Air
Green Flash
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Refraction in Air
Red Flash
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Refraction by Ice Crystals
22 1/2˚ Halo
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Refraction by Ice Crystals
Sun Dogs
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Refraction by Ice Crystals
Sun Dog
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Refraction by Ice Crystals
22 1/2˚ Halo
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Refraction by Ice Crystals
22 1/2˚ Halo and Upper Tangent Arc
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Refraction by Ice Crystals
22 1/2˚ Halo
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Refraction by Ice Crystals
Halo Complex
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Refraction by Ice Crystals
Halo Complex
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Refraction by Ice Crystals
46˚ Halo?
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Refraction by Ice Crystals
Lower Tangent Arc
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Refraction by Ice Crystals
Lower Tangent Arc
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Refraction and Scattering
Sun Pillar and Sun Dog?
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Atmospheric Optics
• Scattering• Reflection• Refraction • Diffraction
The amazing variety of optical phenomena observed in the atmosphere can be explained by four physical mechanisms.
73
Rayleigh and Mie Scattering
Things to look for• Blue Sky• White Clouds• Blue Smoke• Red Sunsets• Crepuscular Rays• Heiligenschein
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Rayleigh scattering by air molecules results in a blue sky. Mie scattering by droplets results in white/grey clouds.
Scattering by Cloud Particles
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When the sun sets or rises, the sunlight passes through a long path of air. Most of the blue light is Rayleigh scattered out, leaving red light, which is Mie scattered toward the observer by clouds.
Scattering by Cloud Particles
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Reflection
• Sun Pillars• Circumhorizontal Arcs
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Refraction
• Green Flash• Mirage• Halo• Tangent Arc• Rainbow
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Refraction
Light slows down as it passes from a less dense to a more dense medium.As light slows it bends toward the denser medium. Similar to waves approaching a beach.The amount of bending depends on the wavelength (color) of the light, leading to dispersion or separation of colors.
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Refraction Causes
Halo Complex results from refraction by ice crystals
MirageGreen FlashHalosSundogsRainbows
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Refraction by Water Drops
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Refraction by Water Drops
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Refraction by Rain
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Refraction by Water Drops
Primary Rainbow
Violet
Red
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Refraction by Rain
Double Rainbow
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Refraction by Rain
Double Rainbow
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Partial Rainbow
Refraction by Rain
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Double Rainbow
Refraction by Rain
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High Sun: Low Rainbow
Refraction by Rain
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Refraction by Rain
Rainbow seen from Airplane
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Diffraction
Constructive interference of light waves can produce color separation.The physical mechanism in this case is called diffraction.Produces colors on soap films, oil slicks, and music CDs.
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Diffraction causes Interference
Diffraction is the tendency of light waves to bend as they pass the edges of objects.
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Diffraction by Soap Film
Diffraction Results in• Iridescence• Corona• Glory• Supernumerary bows
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Diffraction by Cloud Drops
Iridescence
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Diffraction by Airplane Window
Artificial Iridescence
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Diffraction by Cloud Droplets
Corona
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Diffraction by Pollen
Corona
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Diffraction and Reflection
Glory
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Diffraction and Reflection
Glory
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Diffraction and Reflection
Glory
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Diffraction and Reflection
Supernumerary Rays
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Diffraction and Reflection
Fog Bow: larger drops do not allow dispersion of colors.
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Diffraction and Reflection
Fog Bow: larger drops do not allow dispersion of colors.
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Summary: Atmospheric Optics
• Scattering• Reflection• Refraction • Diffraction
The amazing variety of optical phenomena observed in the atmosphere can be explained by four physical mechanisms.
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