Black body radiation

Black bodies in addition to emitting the highest energy at all wavelengths also completely absorb radiation of all wavelengths falling on them. Radiation emitted by a black body is a function of its temperature only. However, there are no perfect black bodies. A body that is closet to a perfect blackbody is cavity having a small hole on the surface. The wall of the cavity is maintained at a constant temperature. All the radiation falling on to the hole gets in to the enclosure and has little or no chance of coming out. That is the radiation falling on the hole is completely absorbed. The radiation that comes out is the radiation that is present in the enclosure. If the enclose is at a temperature T, then the radiation that comes out of the hole has the characteristics of blackbody radiation at temperature T.