What is perfect black body ? Give examples.

Explanation

Perfect black body : ..The body which absorbs all radiant energy incident on it is called perfect black body... Practically it is impossible to have perfectly black body.
For a perfect black body absorptivity `a=1` and emissivity `e=1`.

To obtain perfect black body in practice consider a vessel with a cavity inside it as shown in figure. Inner walls of vessel are irregular and coloured black and a very small hole is provided on its wall.
The radiation incident on this hole experiences multiple reflections inside and at each reflection it gets partially absorbed by its walls.
The possibility of the radiation coming out of the same hole is negligible. Thus, radiation gets absorbed completely inside the vessel.
The part of the wall facing the hole is such that the radiation entering through the hole does not go out of the hle directly after reflection.
In this way, this hole can be regarded as a perfectly black body.
When this vessel is heated uniformly from outside, the radiation emitted through the hole can be considered as the radiation emitted from a perfectly black body. It is called the ..cavity radiation...