Kramers' Opacity Law relates a material's Opacity to its density and Temperature. It applies to high temperatures, e.g., in stars, and applies to Bound-Free (Photoionization) and Free-Free (Bremsstrahlung) Absorption of Visible Light and other Electromagnetic Radiation. It states, that for a material:
Opacity ∝ ρ T-3.5
Since opacity is already defined as the value relating density to the rate of reduction of Intensity, within the regime of Kramers' Opacity law, the actual reduction, and values such as the Optical Depth depend upon the density squared.
Equations have been developed to provide the constant of proportionality for each of the three above types of absorption, which may be summed. Among the elements of these equations are the atomic mass and weight, the density, and the Gaunt Factor, a Wavelength-dependent estimate of some quantum-mechanical contributions to the opacity.
Kramers' Law is a different law, also used in astrophysics, that describes the spectrum of bremsstrahlung X-ray radiation.