text: This quantity's shift under an applied electric field is has a quadratic relation to the electric field in the Kerr effect.
full_question: This quantity's shift under an applied electric field is has a quadratic relation to the electric field in the Kerr effect. The left-hand rule applies in metamaterials, which have negative values for this quantity. This quantity is equal to the square root of the product of relative permeability and relative permittivity. The arc-sine of the ratio of this quantity for two materials at a boundary gives the critical angle where total internal reflection occurs. This quantity for water at 20 Celsius is 1.333. This quantity is multiplied by the sine of the angle of incidence in Snell's law. For 10 points, name this quantity equal to the ratio of the speed of light in a medium to the speed of light in a vacuum. 
first_sentence: This quantity's shift under an applied electric field is has a quadratic relation to the electric field in the Kerr effect.
raw_answer: index of refraction [or refractive index; prompt on "n"]