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For the radiation from a black body in thermal equilibrium, see Black-body radiation. As the temperature of a black body decreases, its intensity also decreases and its peak moves to longer wavelengths. Jeans law and its ultraviolet catastrophe.
A black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. It is an ideal emitter: at every frequency, it emits as much or more thermal radiative energy as any other body at the same temperature. It is a diffuse emitter: the energy is radiated isotropically, independent of direction. An approximate realization of a black surface is a hole in the wall of a large enclosure.
Any light entering the hole is reflected indefinitely or absorbed inside and is unlikely to re-emerge, making the hole a nearly perfect absorber. The radiation confined in such an enclosure may or may not be in thermal equilibrium, depending upon the nature of the walls and the other contents of the enclosure. Real materials emit energy at a fraction—called the emissivity—of black-body energy levels. A source with lower emissivity independent of frequency often is referred to as a gray body.