Properties of Electromagnetic Radiation

Abstract

Light represents one form of electromagnetic radiation, which can be described by a wave theory. Through the scientific history of the sixteenth to nineteenth century a solid knowledge has been developed on electromagnetic radiation, culminating in the classical theory of James Clerk Maxwell (1864) coupling electric with magnetic fields, and describing together with the equation of continuity the interaction of electromagnetic radiation with matter. We start describing first looking at the principal forces acting during the interaction of laser radiation with matter. Thereby we will describe the common fundamental forces. In the following, the Maxwell equations in the differential and in the integral form will be discussed. These equations are for this book of central importance allowing to describe scattering, reflection, and refraction of radiation. As typical fields, the electric and the magnetic field strengths will be introduced. As shown in classical mechanics, it is convenient to define a potential to a force allowing an easier calculation of the interaction. Also for the two electromagnetic field strengths, a scalar and a vector potential will be introduced. In the following of this chapter, we talk about the energy density given by the electromagnetic field getting at the end an idea of how electromagnetic radiations transports energy, namely by the Poynting vector. Using exemplary planar waves, the orientation of the fields as well as the energy density are defined, and at the end also, the important connection between field strength and intensity of radiation is defined. As laser radiation must not be monochromatic, the velocity of light, the so-called phase velocity, is discussed for temporal limited radiation pulses getting the definition of the group velocity. Up to now we worked only with the wave description of photons. Here we start to have a look at its particle character. Finally, we talk about the special properties of laser radiation delimiting from the conventional radiation. So the property of coherence will be discussed here briefly.