Black body: A body having the capability to absorb all the heat radiations of all wavelength completely, incident on it, is known as perfectly black body. But in practice, no materials have the property of absorbing all radiations. For example, lamp-black is approximated as black body, because it absorbs 98% of incident radiation.
When radiant energy is incident on the surface of a body, a portion of energy is reflected, portion is absorbed and rest part is transmitted. If the portions are r, a and t respectively, then for the body, r + a + t =1 Now, if r = o, t = o, then a = 1, the body is then called perfectly black body.
Black Body Radiation:
Ferry design a black body, as shown in figure, which is a hollow metallic sphere having two concentric walls with a fine opening 0. A pointed projection is made in the inner wall against the opening 0. So that, no radiant heat can be liberated out through the opening. When the radiations enter the hole, they suffer multiple reflections and are completely absorbed. This fine hole opening then behaves like a good absorber. When the system is heated uniformly in a bath at constant temperature; the opening remits heat radiation. This radiation from the fine opening is called black body radiation. It should be remembered that only the hole (opening) and not the walls of the body acts as the black body radiator.
[The inner surface of the sphere is made black with lamp-black and outer surface is made polished with Ni-polished.]