An approximate law of scattering of electromagnetic waves by molecules and particles small compared with the wavelength of the illumination at wavelengths for which absorption is sufficiently small. According to this law, first derived in 1871 by Lord Rayleigh using simple dimensional arguments, scattering in all directions by an object is inversely proportional to the fourth power of the wavelength of the illumination. Scattering of sunlight by air molecules does not obey this law exactly, although it is a good approximation. Rayleigh's scattering law also predicts that scattering by a particle is proportional to the square of its volume. Compare Mie theory.