ABSTRACT
A modern example of the connection between emission and absorption is a lightweight thermal blanket with a highly reflective metallic coating that absorbs little, i.e., it mainly reflects radiation. Such a blanket also emits relatively little radiation and is definitely not a blackbody. The photons of blackbody radiation span a wide range of energies. Because photons have wave as well as particle properties, under certain circumstances, they can interfere constructively or destructively with one another. Yet photons in a photon gas have sufficiently randomised directions, wavelengths, and phases that they do not ordinarily affect one another's energies or directions of motion. This chapter describes the thermodynamic functions and reversible Carnot cycle for a photon gas. One of the most inviting examples of a photon gas is the cosmic microwave background radiation (CMBR). Awareness of the photon gas opens the door to an understanding and appreciation of the CMBR.
