Snell's Law: ni sin θi = nr sin θr where ni = refractive index of the first medium,
θi = angle of incidence,
nr = refractive index of the second medium,
θr = angle of refraction.
When light travels from one medium to another medium of lower refractive index and the angle of incidence is greater than a certain critical angle, total internal reflection occurs.
Refractive Indices of Common Materials: Vacuum: 1
Air: 1.0003
Water: 1.33
Glycerine: 1.47
Glass: 1.52
Emerald: 1.58
Ruby: 1.76
Sapphire: 1.77
Diamond: 2.42