perfectly black body:

 A perfectly black body: A body which can absorb all the radiant heat incident on it is called perfectly black body.

1. It consists of doubled walled hollow sphere having small aperture through which heat energy enters.

2. It has conical projection diametrically opposite to the aperture to avoid reflection along same path.

3. The inner surface of sphere is coated with lamp black and outer surface is nickel polished. 4. The space between two walls is evacuated to minimize heat loss by conduction and convection.The radiant energy entering the sphere suffers multiple reflections.

5. During each reflection 98 % of heat is absorbed so that within few reflections all the radiant heat i absorbed by sphere.