| Astrophysics (Index) | About |
An absorption coefficient is a measure of how much electromagnetic radiation (EMR) (e.g., light) is absorbed by a substance as the EMR passes through it. It is used in modeling the effect of gas on the EMR passing through, such as radiative transfer equations. The coefficient is a ratio of the EMR absorbed per EMR per unit length per unit density of the substance. Its value depends upon the type of substance and its density (i.e., absorption has some non-linearity with regard to density), can depend upon temperature, and can vary over the course of the EMR's travel through a substance.
The term a scattering coefficient is used for an analogous ratio indicating how much of the EMR will be scattered rather than pass through. Attenuation coefficient indicates the combined effect of both (but the term absorption coefficient is sometimes used to mean this.) An emission coefficient is an analogous measure the amount of additional EMR as the EMR is passing through a substance.
An absorption cross section is a similar measure, but for a single absorbing particle. They are related by:
σ = α / N
Opacity is another measure of absorption of a substance, consisting of the absorption coefficient per unit mass, i.e., the absorption coefficient divided by the substance's density.
Note the term absorption coefficient is sometimes used differently: merely as a synonym for opacity as defined on this site. The definitions differ regarding whether the value includes the material's density as a factor (or not), and you have to work out which of these two conflicting definitions is intended.