ABSTRACT
In 1900, Max Planck introduced the concept of the quantum of electromagnetic radiation, later called the photon, into physics. Classically, electromagnetic radiation is considered to be a transverse wave with two polarization directions. Phonons in condensed matter are analogous to photons in electromagnetic radiation in a number of ways. Like photons, phonons are bosons and obey the Planck distribution. The importance of the Planck distribution has been shown in this chapter in treating two important applications: black body radiation and the specific heats of solids. Photons and phonons along with other excitations such as magnons in magnetic materials are governed by the Planck distribution.
