Black Body

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. 
	  = 1
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. 
	  = 1
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:
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