Opacity (or Attenuation Coefficient) is a measure of opaqueness of a material, i.e., to what degree light is absorbed as it passes through. As used in astrophysics, the term specifically means the rate at which the density of a material reduces the Intensity of a beam of light passing through. It varies by Wavelength.
Opacity can occur at values from 0 to ∞. It decreases the intensity as follows (given uniform opacity and density):
Id = I0e-κρd
The opacity and intensity in this equation can be subscripted by ν to specify the intended wavelength.
Since the opacity varies by wavelength, some assumption regarding the spectrum must be made to define an opacity applicable to more than a single wavelength. The Rosseland Mean Opacity is an example.
Kramers' Opacity Law
Optical Depth (τ)
Rosseland Mean Opacity
Equation of Radiative Transfer (RTE)
Synthetic Field Method (SFM)