2. Thermal radiation

2.1 Origin

According to the laws of thermodynamics, any body placed in an isothermal enclosure at a non-zero temperature T exchanges radiation with the enclosure, eventually reaching this temperature. With respect to radiation coming from its environment, the body exhibits, on its surface or in its volume, various types of behavior, such as absorption, reflection, transmission and diffusion, which can be defined as follows:

• On the one hand, absorption is the ability of a material's atoms and molecules to trap a fraction of incident photons, transforming the corresponding energy into heat;

• on the other hand, reflection, transmission and scattering are interaction processes between light and matter, each...