Interaction of Radiation with Matter

Abstract

Let us consider a cavity which is filled with a homogeneous and isotropic dielectric medium. If the walls of the cavity are kept at a constant temperature T, they will continuously emit and receive power in the form of electromagnetic (e.m.) radiation. When the rates of absorption and emission become equal, an equilibrium condition is established at the walls of the cavity as well as at each point of the dielectric. This situation can be described by introducing the energy density ρ, which represents the electromagnetic energy contained in unit volume of the cavity. Since we are dealing with electromagnetic radiation, the energy density can be expressed as a function of the electric field E(t) and magnetic field H(t) according to the well-known formula

$$\rho =\frac{1}{2}\in {{E}^{2}}\left( t \right)+\frac{1}{2}\mu {{H}^{2}}\left( t \right)$$

(2.1)

where ε and μ are, respectively, the dielectric constant and the magnetic permeability of the medium inside the cavity.