# Chapter 6

# Interaction of Radiation and Matter

The general theory of the interaction between electromagnetic radiation and matter lies in the realm of Quantum Electrodynamics, which is well beyond the scope of this text. Thus, our aim in this chapter is to formulate a working description of electromagnetic processes, which permits discussion of various photon scattering and absorption processes on a common footing using propagator theory.

## 6.1 A Charged Particle in an Electromagnetic Field

A particle with charge *e* and mass *m* moving under the influence of an electromagnetic field experiences the force field

where is the velocity of the particle and *c* is the speed of light in vacuum. The electric field strength and the magnetic field strength can be expressed in terms of the vector potential and the scalar potential , where is the position vector of the particle. Note that the dependence ...