Black body by definition is an object that absorbs all radiation falling on it or in other words it has an
absorptivity equal to unity.
therefore there is no reflection from a black body. Applying the Kirchoff's law to a
black body results in that it should have an emissivity of unity i.e.
This means that a good absorber of radiation is also a good emitter of radiation. There are no totally black
bodies in practice, but many surfaces approximate to the definition.
A close approximation to a black body is a hole leading to a chamber. Thermal radiation entering the hole is absorbed almost completely by the walls of the chamber, therefore only a small fraction is emitted from the hole. Related topics: