As the temperature of a black body decreases, its intensity also decreases and laws of radiation pdf peak moves to longer wavelengths. 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.
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. Construction of black bodies with emissivity as close to one as possible remains a topic of current interest. This is true for radiation of all wavelengths and for all angles of incidence. This section describes some concepts developed in connection with black bodies.