Q.What is perfectly black body? Explain Ferry's black body.

A perfectly black body absorbs all incident radiation of every wavelength; Ferry's design realises this ideal with a nearly-closed cavity.

Perfectly black body: An ideal body that absorbs all the electromagnetic radiation of every wavelength incident on it, reflecting or transmitting none. Since it is a perfect absorber, by Kirchhoff's law of radiation it is also the best possible emitter of thermal radiation at any given temperature — its emissive power at every wavelength is the maximum theoretically possible for that temperature. No real substance is perfectly black, but several designs closely approximate one.

Ferry's black body: A practical realisation of a black body, consisting of a hollow double-walled sphere (or cylinder), blackened on the inside, with a small opening (aperture) on one side and a conical projection on the inner wall directly opposite the opening. Radiation entering through the small hole undergoes multiple internal reflections and absorptions off the blackened inner walls (and the conical projection, which prevents any direct/first-reflection escape back out of the hole); after each reflection, a large fraction of the remaining energy is absorbed, so that by the time any radiation would find its way back out, its intensity is negligible. The small aperture thus behaves, for all practica …