black body

In physics, a hypothetical object that completely absorbs all electromagnetic radiation striking it. It is also a perfect emitter of thermal radiation.

Although a black body is hypothetical, a practical approximation can be made by using a small hole in the wall of a constant-temperature enclosure. The radiation emitted by a black body is of all wavelengths, but with maximum radiation at a particular wavelength that depends on the body's temperature. As the temperature increases, the wavelength of maximum intensity becomes shorter (see Wien's displacement law). The total energy emitted at all wavelengths is proportional to the fourth power of the absolute temperature (see Stefan-Boltzmann law). Attempts to explain these facts failed until the development of quantum theory in 1900.