as the temperature of a black body decreases, its intensity also decreases and its peak moves to longer wavelengths. shown for comparison is the classical rayleigh–jeans law
and its ultraviolet catastrophe
a black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. (it does not only absorb radiation, but can also emit radiation. the name "black body" is given because it absorbs radiation in all frequencies, not because it only absorbs.) a white body is one with a "rough surface that reflects all incident rays completely and uniformly in all directions."
a black body in thermal equilibrium (that is, at a constant temperature) emits electromagnetic radiation called black-body radiation. the radiation is emitted according to planck's law, meaning that it has a spectrum that is determined by the temperature alone (see figure at right), not by the body's shape or composition.
an ideal black body in thermal equilibrium has two notable properties:
- 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: measured per unit area perpendicular to the direction, the energy is radiated isotropically, independent of direction.
an approximate realization of a black surface is a hole in the wall of a large insulated enclosure (an oven, for example). any light entering the hole is reflected or absorbed at the internal surfaces of the body and is unlikely to re-emerge, making the hole a nearly perfect absorber. when the radiation confined in such an enclosure is in thermal equilibrium, the radiation emitted from the hole will be as great as from any body at that equilibrium temperature.
real materials emit energy at a fraction—called the emissivity—of black-body energy levels. by definition, a black body in thermal equilibrium has an emissivity of ε = 1. a source with lower emissivity independent of frequency often is referred to as a gray body.
construction of black bodies with emissivity as close to one as possible remains a topic of current interest.
in astronomy, the radiation from stars and planets is sometimes characterized in terms of an effective temperature, the temperature of a black body that would emit the same total flux of electromagnetic energy.