Rayleigh scattering is the scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the radiation. It is named after Lord Rayleigh, who first proposed a mathematical theory of this effect in 1871. Rayleigh scattering usually occurs when the radiation is scattered by particles with a size much smaller than the wavelength of the radiation, such as molecules of air or particles in a suspension. This type of scattering results in a decrease in intensity and color shift in the scattered light, which is why the sky appears blue and sunsets appear red.