Description of Polarized Light

Abstract

A medium like a planetary atmosphere or ocean contains electromagnetic radiation. This situation is commonly referred to by saying that a radiation field exists in the medium. We will make the basic assumption of the classical theory of radiative (energy) transfer, namely that energy is transported across surface elements along socalled pencils of rays. A fundamental quantity in the description of a radiation field is the intensity at a point in a direction. It may be defined as follows. The amount of radiant energy, dE, in a frequency interval (v, v+ dv) which is transported in a time interval dt through an element of surface area dσ and in directions confined to an element of solid angle dΩ, having its axis perpendicular to the surface element, can be written in the form

$$dE = Idvd\sigma d\Omega dt,$$

((1.1))

where I is the (specific) intensity [See Fig. 1.1] . The intensity allows a proper treatment of the directional dependence of the energy flow through a surface element. In practice, it may be determined from a measured amount of radiant energy by letting dv, dσ, dΩ and dt tend to zero in an arbitrary fashion. In most media the intensity not only depends on the point but also on the direction considered. Loosely speaking we may say that the intensity I of a radiation field at a point O in the direction m of a unit vector m is the energy flowing at O in the direction m, per unit of frequency interval, of surface area perpendicular to m, of solid angle and of time.