For 10 points, name this quantity equal to the speed of light in a vacuum over its speed in a given material.
Imaginary values of this quantity correspond to a phase shift between applied and resultant electric field in a material. This quantity is related to a material's density and polarizability in the Lorentz-Lorenz equation, and the OPL is equal to this quantity times the distance travelled by light. At (*) angles greater than the arcsine of the ratio of two of these quantities, light experiences total internal reflection. Snell's law equates the ratio of this value for two substances to the ratio of the sines of the angles of incidence and refraction. For 10 points, name this quantity equal to the speed of light in a vacuum over its speed in a given material.
Imaginary values of this quantity correspond to a phase shift between applied and resultant electric field in a material.
index of refraction (accept refractive index or n)