dispersion of electromagnetic radiation by particles that have a radius less than approximately ¹/₁₀ the wavelength of the radiation. The process has been named in honour of Lord Rayleigh (Rayleigh, John William Strutt, 3rd Baron), who in 1871 published a paper describing this phenomenon.

The angle through which sunlight in the atmosphere is scattered by molecules of the constituent gases varies inversely as the fourth power of the wavelength; hence, blue light, which is at the short wavelength end of the visible spectrum, will be scattered much more strongly than will the long wavelength red light. This results in the blue colour of the sunlit sky, since, in directions other than toward the Sun, the observer sees only scattered light. The Rayleigh laws also predict the variation of the intensity of scattered light with direction, one of the results being that there is complete symmetry in the patterns of forward scattering and backward scattering from single particles. They additionally predict the polarization of the scattered light.