Perfectly Black Body

Perfectly Black Body

A body which absorbs the entire radiant heat incident upon it is called a perfectly black body. Thus, the coefficient of absorption of a perfectly black body is equal to 1. 

1) A perfectly black body is that which absorbs completely the radiations of all wavelength incidents on it.

2) As a perfectly black body neither reflects nor transmits any radiation, therefore the absorptance of a perfectly black body is unity, i.e. t = 0 and r = 0 \(\Rightarrow \) a = 1

3) We know that the color of an opaque body is the color of radiation reflected by it. As a black body reflects no wavelength so, it appears black, whatever be the color of radiations incident on it.

4) When perfectly black body is heated to a suitable high temperature, it emits radiation of all possible wavelengths. For example, temperature of the sun is very high; it emits all possible radiation so it is an example of black body.

5) Ferry’s black body: A perfectly black body can’t be realized in practice. The nearest example of an ideal black body is the Ferry’s black body. It is a doubled walled evacuated spherical cavity whose inner wall is blackened. The space between the walls is evacuated to prevent the loss of heat by conduction and radiation. There is a fine hole in it. The entire radiations incident upon this hole are absorbed by this black body. If this black body is heated to high temperature then it emits radiations of all wavelengths. It is the hole which is to be regarded as a black body and not the total enclosure.

6) A perfectly black body cannot be realized in practice but materials like Platinum black or Lamp black come close to being ideal black bodies. Such material absorbs 96% to 85% of the incident radiations.