Rayleigh scattering is a physical phenomenon where light is scattered in different directions by very small particles. These particles are much smaller than the wavelength of the light involved and may even be as small as a single atom. Rayleigh scattering is most commonly seen in gases although it can occur in both liquids and solids. The amount of scattering present depends on the polarizing properties of a particular type of particle and can vary depending on the elements involved.
The amount of scattering is dependent on a number of factors. For scattering to be known as Rayleigh scattering, the particle must be much smaller than the wavelength of light &mdahs; if the particle is close to the size of the wavelength then the approximations used for Rayleigh scattering are no longer correct. The larger the particle, however, the greater the intensity of scattering, while the larger the wavelength the less scattered intensity.
Light can be scattered from both atoms and molecules. When an atom or single particle is involved, then the small size approximation can be used. This uses several assumptions, mainly that the particle is very small and that the radius of the particle and refractive index can be measured. A molecule often doesn’t have such a clearly defined radius and so slightly different formulas have to be used in order to work out the intensity of scattering. In this case the polarizability — the amount that the charge on the molecule will be affected by an electric field — is used to work out the scattering intensity.
Rayleigh scattering is the mechanism that causes the sky to be blue. When sunlight travels through the atmosphere, it is scattered by particles that are present. Some wavelengths of light, however, are scattered more than others. In this case, blue light is scattered more efficiently and the sky appears blue most of the time. The only exception is during sunset or sunrise where the sun’s rays are passing directly through the atmosphere. In this case, the sky appears redder as red light isn’t scattered as much as blue and can pass through the atmosphere unaffected.
There are also several practical applications of Rayleigh scattering that are used in modern technology. For example, the fact that light is scattered is used in some optical fibers. For the optical fibers to function correctly there needs to be some scattering of the optical signals and this is achieved using small particles.