Question: A spherical black body is kept at constant temperature thermal equilibrium T and by supplying heat from some source. It is covered by another spherical surface which is also a black body (a very thin surface of radius almost equal to the previous one). Find the temperature at which the system is now in thermal equilibrium. Assume that heat radiated goes into free space. Can we use the concept that heat lost by the inner black body is equal to heat gained by it??? If not, then how to attempt the problem?