In a broad sense, the process by which matter is excited to radiate by an external source of electromagnetic radiation, as distinguished from emission of radiation by matter, which occurs even in the absence of such a source. By this definition, reflection, refraction, and even diffraction of electromagnetic waves are subsumed under scattering. Sometimes scattering is applied in a restricted sense to that radiation not accounted for by the laws of specular reflection and refraction, which are approximate because matter is not continuous on all scales. Often the term scattered radiation is applied to that radiation observed in directions other than that of the source and may also be applied to acoustic and other waves. If there is no change in frequency between the incident and scattered radiation, the scattering is sometimes said to be elastic; the converse is inelastic. Scattering is also applied to any interaction between particles that results in a change in direction. See multiple scattering, Mie theory, Rayleigh's scattering law.