Atmospheric refraction

Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of altitude. Atmospheric refraction near the ground produces mirages and can make distant objects appear to shimmer or ripple.

Related Topics:
Light - Electromagnetic wave - Mirage

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Atmospheric refraction causes astronomical objects to appear higher in the sky (up to half an arcminute at visible wavelengths for objects close to the horizon) than the actual direction from which the light came. Telescope control systems are usually designed to take the refraction correction into account so that targets can be observed anywhere in the sky. The amount of refraction varies as a function of the observing wavelength, causing astronomical objects to be spread out into a spectrum in high-resolution images. In order to overcome this, atmospheric refraction correctors are usually employed for broadband high-resolution observations (made from pairs of rotating glass prisms). The amount of atmospheric refraction from air is highest at the blue end of the visible spectrum, and decreases in the red and infrared. At mid-infrared wavelengths the effects of water vapour become significant, making refraction correction much more complex.

Related Topics:
Astronomical object - Arcminute - Telescope

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Atmospheric refraction can also distort the shape of the sun during sunrise and sunset and cause chromatic aberrations in the sun's image.

Related Topics:
Sunrise - Sunset - Chromatic aberration

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