May 14, 2012

Pre-sunrise colors, and the yellowish ball of fire

click photo for full-size image
photo by Donald Kinney

No finer fun than to copy and paste stuff out of Wikipedia. Makes me sound like I might know what I'm talking about. I don't, but here goes...

Rayleigh Scattering not only produces the bright colors we see in sunrises and sunsets, but also the blue of the sky and the yellow of the sun.

click photo for full-size image
photo by Donald Kinney

Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through transparent solids and liquids, but is most prominently seen in gases. Rayleigh scattering is a function of the electric polarizability of the particles.

Rayleigh scattering of sunlight in the atmosphere causes diffuse sky radiation, which is the reason for the blue color of the sky and the yellow tone of the sun itself.

A portion of the light coming from the sun scatters off molecules and other small particles in the atmosphere. It is this scattered light that gives the sky its brightness and its color. Rayleigh scattering is inversely proportional to the fourth power of wavelength, so that shorter wavelength violet and blue light will scatter more than the longer wavelengths (yellow and especially red light). The resulting color, which appears like a pale blue, actually is a mixture of all the scattered colors, mainly blue and green. Violet, though strongly scattered, is a minor component of the solar spectrum and is less efficiently detected by the human eye. Conversely, glancing toward the sun, the colors that were not scattered away — the longer wavelengths such as red and yellow light — are directly visible, giving the sun itself a slightly yellowish hue. Viewed from outer space, however, the sky is black and the sun is white.

The reddening of sunlight is intensified when the sun is near the horizon, because the volume of air through which sunlight must pass is significantly greater than when the sun is high in the sky. The Rayleigh scattering effect is therefore increased, removing virtually all blue light from the direct path to the observer. The remaining unscattered light is mostly of a longer wavelength, and therefore appears to be orange.

Rayleigh scattering primarily occurs through light's interaction with air molecules. Or, from a purely macroscopic point of view, blue sky comes from microscopic density fluctuations, resulting from the random motion of molecules composing the air. A region of higher or lower density has a slightly different refractive index from the surrounding medium, and therefore it acts like a short-lived particle that can scatter light in random directions. Smaller regions fluctuate more than larger ones, and, since short wavelengths are disturbed by small regions more than longer wavelengths, they are scattered more. (source: Wikipedia)

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1 comment:

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