Opacity -- from Eric Weisstein's World of Physics

Thermodynamics "> Radiative Transfer v

Opacity

The opacity of a medium has units of is in cm² g^-1 and is defined by

where K is the extinction coefficient, is the density, is the cross section, n is the number density.

Chandrasekhar Mean Opacity, Extinction Coefficient, Radiative Transfer Equation, Rosseland Mean Opacity