When electromagnetic radiation impinges on a surface, it can be admitted to the surface, i.e. absorbed. The absorption α describes the ratio of electromagnetic radiation absorbed by an object to the complete incident radiation. The absorption can have a value between 0 and 1, and depends on the materials, the surface condition as well as the radiation angle of incidence and the frequency of the electromagnetic radiation.
The incident radiation can also be reflected from the surface or transmitted through the body. Therefore the absorption is connected to the reflectivity ρ and the transmissivity τ according to the equation:
1 = α + ρ + τ
In the case of a black body, neither reflection nor transmission take place. According to Kirchhoff’s radiation law, for a body in thermal equilibrium: Absorption = emission.