part 7. special topics and appendices chapter 17 atmospheric optics
Post on 17-Dec-2015
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Refraction causes a distortion of images
Refraction is the change in the propagation direction of light due to a change in the velocity of the light. The thicker the atmosphere, the slower the speed of light through the atmosphere.
The light ray turns downward due to the thicker atmosphere at lower elevations
Refraction and the Setting or Rising SunRefraction greatest with low solar angles because the radiation is passing through the thicker lower atmosphere at a great angle from the vertical
The real sunset/sunrise position is different from the one that is visible; refraction also affects the apparent shape of the sun• The color of the sun at sunrise/sunset is
affected by Rayleigh and Mie scattering
Effects of refraction on the solar disk at sunrise or sunset
The sun is actually below the horizon when we first see it at sunrise or last see it at sunset!
Mirages• Mirages arise due to refraction from steep near-
surface temperature gradients causing density changes in the air
• Distant objects appear lower than they actually are
• Further objects appear distorted • Very far objects disappear• Inferior mirages
– A wavy inverted image of the actual image• Superior mirage
– Images displaced upward
Temperature effects and mirages
Person at A is fully seen.Only top half of person at B is seen.The person at C is not seen.
An inferior duplicate mirage
In the inferior mirage, the tree appears to be upside down and below the Earth’s surface. It will appear to be moving like the surface of water on a pond.
RainbowsRefraction and reflection of sunlight entering a raindrop• Droplets in front, sun behind the viewer • Primary rainbows
– Brightest and most common– Shortest wavelengths occupy innermost
portion of the rainbow– Secondary rainbow surrounds primary– A partial rainbow may appear
Rainbow formation
The sun is directly behind the viewer, who is looking at the drops
Secondary rainbows form due to light that reflects twice within the drops
Halos, Sundogs, and Sun Pillars• From cirrostratus clouds• Ice crystals refract light • Halos form around the Sun or Moon
– Radii of 22o or 46o
• Sundogs– Whitish spots in the sky– They occur when ice crystals of 30 m
align horizontally
Sun pillars • Ice crystals reflect sunlight off their tops and bottoms
Coronas and glories• The form from light bending around water droplets
due to diffraction (light changing direction around an object)
• A corona is a circular illumination surrounding the Moon or Sun due to clouds with uniform droplet sizes Circular illumination surrounding the Moon or Sun due to clouds with uniform droplet sizes
• A glory is a rainbow seen on clouds from above (such as while flying in aircraft)
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