ABSTRACT
This chapter deals with the optical theorem which relates the intensity of light that is scattered by a particle into exactly the forward direction to its extinction cross section. It shows how to compute the scattering and absorption coefficients of particles. The chapter is concerned with a strange but important property of the material constants that appear in Maxwell’s equations, such as ε or μ. The optical theorem asserts that the reduction of intensity in the forward direction fully determines the particle’s extinction cross section. The cross section of composite particles can be computed exactly in those few cases where the components are homogeneous and the geometrical structure is simple; examples are spherical shells, cylinders with mantles or coated ellipsoids. The new big grain has a fluffy structure and for its description, even if the grains before coagulation were homogeneous and chemically identical, at least one additional dielectric function is needed, namely that of vacuum.
