ABSTRACT
In this chapter, the authors have collected the tools to calculate how an electromagnetic wave interacts with a spherical grain. With the help of a small computer, the formulae presented so far allow to derive numbers for the cross sections, scattering matrix, phase function, and so forth. To the student who wishes to perform similar calculations, the figures also offer the possibility of checking his computer program against the one used in this text. As a first illustration, the authors consider the scattering efficiency of a non-dissipative sphere, i.e. one that does not absorb the incident light. Impurities effectively suppress the ripples and they also lead to a decline in the amplitudes of the waves. For interstellar grains, the ripples and the waves are irrelevant. As preferentially blue light is removed in scattering, the transmitted light becomes redder. More relevant for the reddening towards the horizon is the removal of blue light through absorption by tiny solid particles.
