For substances with varying values for this quantity, one can use the Fresnel equations.
The Lorentz-Lorenz or Clausius-Mosotti equation for a gas ties this quantity to temperature and pressure. This quantity is squared on the left side of the Sellmeier equations, which relate it to wavelength.  For substances with varying values for this quantity, one can use the Fresnel equations. It is approximated as the square root of the product of the relative permeability and permittivity and it is equal to the speed of light divided by the phase velocity of a dispersive material. For 10 points, name this quantity which appears in Snell's Law, measuring the reduction of the speed of light in a material.
The Lorentz-Lorenz or Clausius-Mosotti equation for a gas ties this quantity to temperature and pressure.
Index of {Refraction} or {Refractive} Index