For 10 points, name this ratio of the speed of light in a vacuum to the speed of light in a medium.
For an oscillating electric field, the square of this quantity can be expressed as the electrical permittivity plus the quantity four-pi-i times the electrical conductivity divided by the angular frequency. The Fizeau experiment showed that in luminiferous aether theory, the Fresnel drag coefficient for propagation of light in a medium is equal to one minus one over this quantity squared. This quantity experiences a shift that is linear or quadratic in the applied electric field in the Pockels and Kerr effects, resulting in birefringence. Fermat's principle of least time can be used to derive a relation in which a ratio of values of this quantity equals a ratio of sines of angles; that law is Snell's law. For 10 points, name this ratio of the speed of light in a vacuum to the speed of light in a medium.
For an oscillating electric field, the square of this quantity can be expressed as the electrical permittivity plus the quantity four-pi-i times the electrical conductivity divided by the angular frequency.
index of refraction [or refractive index]