Black Body:
A body that absorbs radiations of all wavelengths falling on it, without reflecting or transmitting any of it. It is called the ideal black body. As it is an ideal absorber it is an ideal emitter as well. At a particular temperature, energy radiated by a black body is greater than that radiated by any other substance. Hence, radiation emitted by an ideal black body is called total radiation or black body radiation.
In reality, there is no ideal black body, though lamp black and platinum black are considered to be nearly ideal. lamp black absorbs about 96% and platinum black absorbs about 98% of the radiation failing on them.
Ferry’s Black Body:
A hollow double-walled metal sphere kept at a steady te, temperature with a fine hole behaves like an ideal black body. It is blackened inside with a lamp black and nickel-polished on the outside. The space between the two walls doesn’t have any air. So, heat isn’t lost due to conduction and convection. Heat radiation entering the sphere through the small opening O gets completely absorbed inside the sphere after a few successive reflections. The projection P, opposite to the opening O prevents normal reflection of the radiation.
So, no radiation enering the sphere through the opening can come out of the sphere. It means that the total radiation get absorbed. Therefore, the sphere acts as an ideal black body. This is known as Ferry’s Black Body.