ABSTRACT
In section 2.1, we define cross sections, the most important quantities describing the interaction between light and interstellar grains. Section 2.2 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. In section 2.3 to 2.5, we learn how to compute the scattering and absorption coefficients of particles. Section 2.6 is concerned with a strange, but important property of the material constants that appear in Maxwell’s equations, like ε or µ. They are complex quantities and Kramers and Kronig discovered a dependence between the real and imaginary part. In the final section, we approximate the material constants of matter that is a mixture of different substances.
