Perfect black body is one which absorbs heat radiation of all wavelengths, which falls on it. Such a body neither reflects nor transmits any part of the incident heat radiation and hence appears black irrespective of the color of incident radiation. Obviously, the absorbance of a perfect black body.
A perfect black body when heated emits radiation of all possible wavelengths at that temperature. The range of the wavelength of emitted radiation is independent of the material of the body and depends only on the temperature of the black body.
In practice, a surface coated with lamp black or platinum black absorbs about 98% of the heat radiation falling on it. However, on heating, it is not found to emit full radiation spectrum. Therefore, such a body can serve as perfect black body only for absorption of heat radiation. It is found that if a hollow body is heated, the radiation coming out from the inner surface through a small opening in it constitute the full radiation spectrum. Such radiation is called cavity radiation. Such a body acts as a perfect black body both for absorption and emission.
Emissivity of a body
The Emissivity of a body is defined as the ratio of the heat energy radiated per second per unit area by the given body to the amount of heat energy radiated per second per unit area by a perfect black body at the same temperature.
It is represented by e. obviously, for a perfect black body its value is 1. The emissivity of polished copper, polished aluminum and lamp black are 0.018, 0.05, and 0.95 respectively.