An Etymological Dictionary of Astronomy and Astrophysics

The scattering of light by → particles of size small compared with the → wavelength of light. The intensity of the light scattered by unit volume of the medium at an angle θ to the direction of propagation of the incident light is:

I_θ = 8 π⁴α² N I₀ (1 + cos²θ)/(R²λ⁴),

where α is the → molecular polarizability, N is the number of scattering molecules, I₀ is intensity of the incident light, λ is the wavelength, and R is the distance from the scatterer.

The fourth power dependence on wavelength means that blue light is much more strongly scattered than red light from a medium containing very fine particles.
The air molecules, mostly → nitrogen (78%) and → oxygen (21%) are some 1,000 times larger than → visible light wavelengths.

This accounts for the bluish appearance of smoke and of clear sky when the observation is not along the direction of illumination. The setting Sun, seen through a considerable thickness of atmosphere appears reddish because long wave radiation predominates in the transmitted light.

Historically, John Tyndall first discovered this phenomenon in 1859 (→ Tyndall effect), but Lord Rayleigh studied it in more detail in 1871.

See also: → rayleigh; → scattering.