The deflection of a light ray as it encounters matter while propagating in a physical medium. Named for Lord Rayleigh, a British physicist, the phenomenon is due to the interaction of light and matter at the atomic or molecular level.The closer the size of the particles to the wavelength of the light, the more scattering takes place. As scattering varies as the reciprocal of the fourth power of the wavelength (Scattering = -4 ) the phenomenon decreases rapidly as the wavelength increases. As the light scatters it also variously is absorbed and attenuated by interaction with density changes and compositional variations in the crystalline structure of an optical fiber and the impurities that are always present to some extent. So, the longer wavelengths (e.g., 1550 nm) suffer less attenuation over a distance than the shorter wavelengths (e.g., 850 nm). Rayleigh scattering is the reason that the sky is blue in the day and red at sunset.The shorter blue wavelengths are scattered by matter in the atmosphere more than the green and red wavelengths, so we see blue, rather than the black of space, when the sun is overhead. During the sunset, however, the sun is at such a low angle and the sunlight passes through so much atmosphere that the shorter blue wavelengths are scattered and absorbed so much that we see little of them. The longer red wavelengths suffer less attenuation and, therefore, reach our eyes. See also atmosphere, light, medium, physical, propagation, ray, and wavelength.