Everything about Rayleigh Scattering totally explained
Rayleigh scattering (named after
Lord Rayleigh) is the
scattering of
light or other
electromagnetic radiation by particles much smaller than the
wavelength of the light. It can occur when light travels in transparent solids and liquids, but is most prominently seen in
gases. Rayleigh scattering of
sunlight in clear atmosphere is the main reason
why the sky is blue. Rayleigh and cloud-mediated scattering contribute to diffuse light (direct light being sunrays).
For scattering by particles similar to or larger than a wavelength, see
Mie theory or
discrete dipole approximation (they apply to the Rayleigh regime as well).
Small size parameter approximation
The size of a scattering particle is parametrized by the ratio
x of its characteristic dimension
r and wavelength
λ:
»
The Rayleigh scattering coefficient for a group of scattering particles is the number of particles per unit volume
N times the cross-section. As with all
wave effects, in
incoherent scattering the scattered powers add arithmetically, while in coherent scattering, such as if the particles are very near each-other, the fields add arithmetically and the sum must be squared to obtain the total scattered power.
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 wavelength being scattered to a greater extent than longer wavelengths, and so one sees blue
light coming from all regions of the sky. Direct radiation (from definition) is coming directly from the Sun. Rayleigh scattering is a good approximation to the manner in which light scattering occurs within various media for which scattering particles have small size parameter.
Why is the sky blue?
When one looks at the sky during the day, rather than seeing the black of space, one sees light from Rayleigh scattering off the air. Rayleigh scattering is inversely proportional to the fourth power of wavelength, which means that the shorter wavelength of blue light will scatter more than the longer wavelengths of green and red light. This gives the sky a blue appearance.
Conversely, when one looks towards the sun, one sees the colors that were not scattered away — the longer wavelengths such as red and yellow light. When the sun is near the horizon, the volume of air through which sunlight must pass is significantly greater than when the sun is high in the sky. Accordingly, the gradient from a red-yellow sun to the blue sky is considerably sharper at sunrise and sunset.
While Rayleigh scattering explains the blue color, there wouldn't be any light at all without some particles to do the scattering. Aside from that which occures through light's interaction with air molecules, some of the scattering is also from
aerosols of sulfate particles. For years following large
Plinian eruptions, the blue cast of the sky is notably brightened due to the persistent sulfate load of the
stratospheric eruptive gases. Another source of scattering is from microscopic density fluctuations, resulting from the random motion of the air molecules. A region of higher or lower density has a slightly different
refractive index than the surrounding medium, and therefore it acts like a short-lived particle that can scatter light.
Further Information
Get more info on 'Rayleigh Scattering'.
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