The term Absorption is used in the study of Radiative Transfer and Spectroscopy to mean the removal of photons from a beam of Electromagnetic Radiation (e.g., light), e.g., through interactions of photons and particles such as molecules, atoms, ions, or electrons. In this usage, a photon which is scattered out of the beam is considered absorbed. It is the inverse of Emission, the addition of photons to the beam. The concept is used widely in astrophysics, for explaining and modeling stars, for dealing with the effects of the Earth's Atmosphere, for explaining and modeling gas clouds, etc.
Absorption is complicated by the variety of processes that cause it, and varies by the density, the makeup, and the Temperature of the material the beam is passing through. Absorption Lines are a result of a type of absorption that is very sensitive to Wavelength.
A classification of types of absorption is based upon the status of an electron with which the photon is interacting:
The first three have an inverse which constitutes emission. Electron scattering is both absorption and emission, the latter for the beam along the photon's new trajectory.
Distance Modulus (u)
Kramers' Opacity Law
Mean Free Path
Optical Depth (τ)
Radiative Transfer (RT)
Radiative Transfer Code (RT Code)
Equation of Radiative Transfer (RTE)
Radiative Transfer Model (RTM)
Source Function (S)
State of Excitation