One is subtracted from this quantity in the lensmaker's equation.
Optical path length equals the product of distance and this quantity, which is related to density and polarizability in the Lorentz-Lorenz equation. This quantity is approximately equal to the square root of the dielectric permittivity, and some metamaterials have a negative value for it. One is subtracted from this quantity in the lensmaker's equation. Values of it are needed to calculate Brewster's angle as well as the critical angle, above which total internal reflection occurs. This quantity equals 1.33 for water, and it can be calculated using Snell's law. For 10 points, name this quantity, the ratio of the speed of light in a vacuum to the speed of light in a certain medium.
Optical path length equals the product of distance and this quantity, which is related to density and polarizability in the Lorentz-Lorenz equation.
index of refraction [or refractive index]