The term Absorption is used in the study of Radiative Transfer (RT) and Spectroscopy to mean the removal of photons to a beam of Electromagnetic Radiation (EMR) (e.g., light), e.g., through interactions of photons and particles such as molecules, atoms, ions, or electrons. The term includes the removal effects of scattering. 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 because of the many different processes that cause it, and varies by the density, the makeup, 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 the photon is interacting:
The last is also emission, and the other three all have an inverse which is emission.
Absolute Magnitude (M)
Distance Modulus (u)
Kramers' Opacity Law
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