ABSTRACT
(a) To be an exact blackbody a system must be in thermal equilibrium. There can therefore by no flow of heat (no temperature gradients). On the other hand in order to radiate a net energy flux to its environment a body must possess a thermal gradient. An isolated body cannot therefore radiate exactly as a blackbody. Approximate blackbodies are obtained for systems with negligible net fluxes. This can be either because the system has been artificially constructed to trap radiation (as in Kirchoff's box) or because radiation is trapped in the body by absorption and scattering processes.
