Thermodynamics of radiation
This chapter introduces the application of thermodynamics to electromagnetic radiation that's in equilibrium with matter, what's called cavity radiation or the closely-related black body radiation. The term black body as a perfect absorber of radiant energy was introduced in 1860 by Gustav Kirchhoff, who applied thermodynamics to electromagnetic radiation, or heat radiation as it was called then. Cavity radiation can thus be treated as a macroscopic system in equilibrium. Let's calculate the efficiency of a Carnot cycle with the photon gas as the working substance. An important step in that direction, and the last to be taken using thermodynamics, is Wien's displacement law, derived in 1893. Three processes are associated with the interaction of material objects and electromagnetic radiation: reflection, absorption, and emission. Cavity radiation is independent of the specifics of the cavity—the size and shape of the cavity or the material of the walls—and depends only on the temperature of the walls.