When radiant energy falls on the surface of any body, a part of it is reflected, apart is aborted and the rest is transmitted. The whole of the energy is not absorbed because generally the surfaces of ordinary bodies are not perfect absorbers’ of radiation. However if the radiant energy is allowed to fall on  the blackened metallic surface or carbon black, it is found than the energy is almost completely absorbed. A body which completely absorbs the radiant energy falling on it is called a perfectly black body. The absorption is found to be more perfect if we take a hollow sphere blackened on the inside and having a small hole for the entry of the radiation. This is so because any radiation that enters through the hole is reflected over and again by the walls of the sphere till finally is completely absorbed.


         
       Black body due to Fery.
  
The most commonly used black body is the one designed by Fery and is shown in Fig.

It is a hollow double walled metallic sphere having a conical projection P opposite to the hole H and is coated on the inside with lamp black. The projection helps to avoid any direct reflection as otherwise the body will not act as a black body.

Further, a blackbody is not only a perfect absorber of the radiant energy but also a perfect radiator. In fact, of all the bodies, the blackbody radiates the maximum amount of energy for the given temperature. The radiation thus emitted is called black body radiation. ]

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