For two materials, the ratio of the sines of the angles of incidence at an interface equals the ratio of this quantity for both materials, a result that derives from Fermat's principle of least time.
The relationship between this quantity and wavelength is defined by the Sellmeier equation, which models dispersion.  Taking the arctangent of the ratio of this quantity for two materials gives Brewster's angle. For two materials, the ratio of the sines of the angles of incidence at an interface equals the ratio of this quantity for both materials, a result that derives from Fermat's principle of least time. For 10 points, name this physical property that can be determined from Snell's law, defined as the ratio of the speed of light in a given medium to that in a vacuum.
The relationship between this quantity and wavelength is defined by the Sellmeier equation, which models dispersion.  
index of refraction [or refractive index; prompt on n; prompt on IR; do not prompt on partial answers]