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Rayleigh scattering (named after Lord Rayleigh) is
the scattering of light by particles smaller than the wavelength of the light. It occurs when light travels in transparent solids and liquids, but is most prominently
seen in gases. Rayleigh scattering of sunlight from particles in the atmosphere is the reason why the light from the sky is blue.
The amount of Rayleigh scattering that occurs to a beam of light is dependent upon the size of the particles and the
wavelength of the light; in particular, the scattering coefficient, and hence the intensity of the scattered light, varies inversely with
the fourth power of the wavelength, a relation known as the Rayleigh law. The angular intensity polarization
relationships for Rayleigh scattering are conveniently simple. For particles not larger than the Rayleigh limit, there is
complete symmetry in the probability of scattering with respect to a plane normal to the direction of the incident radiation, so
that the forward scatter equals the backward scatter. The Rayleigh scattering coefficient ks is
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where n is the number of scatters of diameter d; m is the index of refraction; and λ is the
wavelength of the radiation.
The strong wavelength dependence of the scattering (~λ-4) means that blue light is scattered much more than red light. In the atmosphere, this results
in blue photons being scattered across the sky to a greater extent than photons of a longer wavelength, and so one sees blue
light coming from all regions of the sky whereas the rest is still mainly coming directly from the Sun.
A notable exception occurs during sunrise and sunset, when the Sun's light must pass through a much greater thickness of the atmosphere to reach an observer on
the ground. This extra distance causes multiple scatterings of blue light, but relatively little scattering of red light; this is
seen as a pronounced red-hued sky in the direction towards the sun.
If the size of particles are larger than the wavelength of light, light is not separated and all wavelengths are scattered as
by a cloud which appears white, as do salt and sugar. For scattering by particles similar to or larger than a wavelength, see the
article on Mie scattering.
See also: Optical phenomenon
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