Some Fundamental Definitions

Abstract

In this initial chapter we introduce some basic definitions about radiation, such as the concepts of luminosity, flux, intensity and radiation energy density. We will explain how the intensity of the radiation is changed by the processes of emission, absorption and diffusion, and associate these processes to the Einstein coefficients and to the random walk. Then the general concepts of thermal and non-thermal plasmas are introduced. A requirement for having thermal plasmas is that particles can exchange energy among themselves. One of the most important ways to achieve that is through Coulomb collisions, and the associated cross section will be derived. Finally, we will examine the electric field of a moving charge, to explain at a fundamental level what radiation is. This will lead to find out the power emitted by an accelerating charge.