A general description of the scattering process

To a greater or lesser extent scattering occurs whenever electromagnetic radiation interacts with matter. Perhaps the best-known example is Rayleigh scattering the results of which are a matter of common everyday observation. The blue of the sky and the haloes which are seen to surround distant lights on a foggy evening are due to the Rayleigh scattering of visible light by molecules of gas or particles of dust in the atmosphere.

The type of scattering we are going to consider can be thought of as due to the absorption of incident radiation with subsequent re-emission. The absorbed incident radiation may be in the form of a parallel beam but the scattered radiation is re-emitted in all directions. The spatial distribution of energy in the scattered beam depends on the type of scattering process which is taking place but there are many general features common to all types of scattering.

In fig. 2.1 the point O represents a scattering centre. The incident radiation is in the form of a parallel monochromatic beam and this is represented in the figure by the bundle of parallel rays. The intensity at a point within a beam of radiation is defined as the energy per unit time passing through unit cross-section perpendicular to the direction of propagation of the radiation. Thus for parallel incident radiation the intensity may be described as the power per unit cross-section of the beam.