1. Semitransparent media

The semitransparency of a material depends on its ability to let photons penetrate more or less deeply before they are absorbed or scattered:

• if the penetration depth is (very) low compared to the dimensions of the medium, the latter is considered to be optically thick, or even opaque: the radiation is ultimately limited to a surface phenomenon;

• conversely, if photons can pass through the medium without being absorbed or scattered (or only in very small numbers), the medium will be considered as optically thin, or even transparent.

A material medium can only absorb a photon if it is capable of transforming the energy it receives:

• either into electronic excitation energy;

• or into vibrational...