### absorption coefficient

(measure of how much light a substance absorbs)

An **absorption coefficient** is a measure of how much
electromagnetic radiation (EMR) (e.g., light)
a substance absorbs as it passes through the substance,
used in modeling the effect of gas on the EMR passing through.
It is a ratio of the EMR absorbed to that entering the
substance over a standard length through the substance.
Its value depends upon the substance and its density.
In some cases, it is specific to the direction the EMR
is passing through or from a specific source, e.g.,
within the substance.

Similarly, a **scattering coefficient** gives such a ratio
indicating how much of the EMR will be scattered rather than
pass through, and an **attenuation coefficient** indicates the
combined affect of both. An absorption coefficient is the inverse
of an emission coefficient.

An **absorption cross section** characterizes the same measure
as the *absorption coefficient*, but for a single absorbing particle.
They are related by:

σ = α / N

- σ - absorption cross section.
- α - absorption coefficient.
- N - number density of particles.

Opacity means roughly the same thing as *absorption coefficient*,
but is generally used to mean the absorption coefficient divided by density,
thus including only any non-linear effects of the density.

(*physics,EMR,measure,coefficient*)
http://en.wikipedia.org/wiki/Attenuation_coefficient

https://www.britannica.com/science/absorption-coefficient

**Referenced by:**

emission coefficient (j)

opacity

oscillator strength

equation of radiative transfer (RTE)

source function (S)

index