If a beam of light illuminates a rough surface, or a cloud of small particles, some of the light is removed from the beam and redistributed in all directions. This angular redistribution is called scattering. The scattered rays go off in many directions different from that of the incident light.
On the other hand, light falling on a polished surface is reflected in a particular direction. The typical example of such a surface is a mirror; so this reflection is called specular (as opposed to diffuse, for scattered light).
Light scattered by the atmosphere makes air visible. This “air light” is mostly scattered by small particles suspended in the air. The absence is scattered light makes cloud shadows visible, producing crepuscular rays.
The small scattering particles suspended in air are called aerosol, so this kind of scattering is called aerosol scattering. The aerosol may be small mineral grains (“dust”) or droplets of salt water. Smog contains aerosol particles produced by photochemical reactions, a result of sunlight shining on hydrocarbon vapors. Smoke and diesel-engine exhaust contain small carbon particles that scatter as well as absorb light.
Most of the aerosol particles are just a few wavelengths of light across, so the wave nature of light must be used to calculate aerosol scattering. As a small obstruction can diffract light much like a pinhole of the same size, much of the scattered light is scattered by diffraction.
However, because the aerosol particles are usually a little bigger than the wavelength of light, they scatter all wavelengths about equally well. The scattered light is, on the average, “white”. That's why clouds are white, and a smoggy sky is whitish or grayish.
Sometimes there's a slight preference for the shorter wavelengths. If the particles are comparable to the wavelength in size, a very fine aerosol can look bluish. A familiar example is cigarette smoke. As “clear air” contains a mixture of aerosol particle sizes, the scattered light usually has a slightly bluish tinge.
But even perfectly clean air still scatters some light. The molecules themselves, though much smaller than the wavelengths of light, can still scatter a little. But, because the molecules are smaller than the wavelength, they scatter short wavelengths (which are more nearly comparable to the size of the molecules) better than long wavelengths.
This scattering by particles much smaller than the wavelength was first studied by Lord Rayleigh in 1871. Because he worked out the details of this process, it's generally called “Rayleigh scattering.”
Rayleigh scattering is the cause of the blue sky: the shortest wavelengths of sunlight (blue and violet) are scattered better than the longer ones, and the average color of the scattered light is the blue of the sky. This color is much more intense than the pale blue of aerosol scattering.
Because the short wavelengths are selectively scattered, the remaining direct sunlight contains mostly the longest wavelengths: red and orange. That's why the setting Sun looks reddish. This removal of light by scattering is the main component of atmospheric extinction.
The preferential scattering of blue light explains why green flashes are usually green, rather than blue or violet. The air scatters the blue and violet so efficiently that they are almost completely removed from the direct ray of light from the Sun; so the last color visible at sunset is usually green, rather than blue or violet. There's usually too little blue to be seen, except in extremely clear conditions.
A little more information about scattering and extinction is available on the introduction to optics page. For a more technical treatment of extinction and scattering, see the extinction page. For more details on the colors of green flashes, see the GF colors page; for examples of how aerosol scattering affects the colors of green flashes, see here and here.
Copyright © 2004 – 2006, 2012 Andrew T. Young
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