CIX. On the scattering gross section of an obstacle

Abstract

Van de Hulst's theorem giving the relation between the scattering cross section and the scattered ‘ amplitude ’ for an obstacle in an incident plane wave is derived by a calculation of the energy flow. In addition it is shown that, when the incident wave is due to a point source, the sum of the scattering and absorption cross sections is determined by the value of a certain field in the neighbourhood of the source. The argument, which is based on energy considerations, could be applied to many scattering problems, e.g. sound waves or atomic collisions but is given explicitly for the electromagnetic field.