(removal of photons from a beam of EMR)
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
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:
- bound-bound - with an electron that is bound to an atom both before and after the interaction: this is the type of absorption that results in spectral lines.
- bound-free - an electron being freed from an atom such as a neutral atom being ionized. This is called the photoelectric effect or photoionization.
- free-free - an electron absorbing a photon, which can only happen when near a charged particle such as an ion.
- electron scattering - Thomson scattering or Compton scattering.
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.
ionized carbon fine structure line ([CII])
distance modulus (u)
emission coefficient (j)
Interstellar Medium Absorption Profile Spectrograph (IMAPS)
Kramers opacity law
mean free path
optical depth (τ)
PHOENIX stellar model
radiative transfer (RT)
radiative transfer code (RT code)
equation of radiative transfer (RTE)
radiative transfer model (RTM)
source function (S)
state of excitation