ABSTRACT
Inside a constant-temperature enclosure, photons are continually absorbed and emitted by the walls. e number of photons in the enclosure is not ˆxed but …uctuates around some average number for any chosen frequency. is …uctuation in the number of photons represents an important diƒerence from the situation in the fermion and boson systems considered in Chapters 13 and 14, where the number of particles N is ˆxed. e chemical potential for photons is not deˆned because there is no constraint on N. It follows that μ should be omitted in the photon distribution, and this is shown in Section 15.3. Photons have spin 1 and are bosons. Because they travel at the speed of light, there are two and not three allowed spin orientations. Classically, electromagnetic radiation is considered to be a transverse wave with two polarization directions.
