For 10 points, name this measure of the speed of light in a material relative to that in a vacuum.
Evanescent waves increase, rather than decrease, in material with a negative value for this quantity, though such materials must be artificially engineered to have a negative permeability. This value appears squared as a function of frequency in the Frank-Tamm formula, which gives the energy emitted by particles traveling faster than the speed of light. The sine of the angle of incidence, divided by that of the angle of transmission, gives the ratio of this quantity for two different materials, via Snell's Law. For 10 points, name this measure of the speed of light in a material relative to that in a vacuum.
Evanescent waves increase, rather than decrease, in material with a negative value for this quantity, though such materials must be artificially engineered to have a negative permeability.
index of {refraction} [prompt on {n}]