<P> where R is the distance to the particle and θ is the scattering angle . Averaging this over all angles gives the Rayleigh scattering cross-section </P> <P> The fraction of light scattered by a group of scattering particles is the number of particles per unit volume N times the cross-section . For example, the major constituent of the atmosphere, nitrogen, has a Rayleigh cross section of 6969509999999999999 ♠ 5.1 × 10 m at a wavelength of 532 nm (green light). This means that at atmospheric pressure, where there are about 7025200000000000000 ♠ 2 × 10 molecules per cubic meter, about a fraction 10 of the light will be scattered for every meter of travel . </P> <P> The strong wavelength dependence of the scattering (~ λ) means that shorter (blue) wavelengths are scattered more strongly than longer (red) wavelengths . </P> <P> The expression above can also be written in terms of individual molecules by expressing the dependence on refractive index in terms of the molecular polarizability α, proportional to the dipole moment induced by the electric field of the light . In this case, the Rayleigh scattering intensity for a single particle is given in CGS - units by </P>

Blue colour of sky due to scattering of light