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 (EMR). 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 (τ) depend upon the density squared.
Kramers' Law is a different law, also used in astrophysics, that describes the spectrum of bremsstrahlung X-ray radiation.