The refractive index is a basic property of material, accurate values of which are often required to interpret various types of spectroscopic data. The index of refraction, n, depends not only on the material of the medium through which light passes, but the wavelength of light. Common values of n are often around 1.6.

A list of refraction indices for a number of common materials that you can plug into Snell's Law:

Vaccuum                  1.000000
Helium                   1.000036 
Hydrogen                 1.000140
Oxygen                   1.000276
Argon                    1.000281
Air                      1.0002926
Nitrogen                 1.000297
Carbon Dioxide           1.000449
Liquid Hydrogen          1.0974
Liquid Nitrogen          1.2053
Water at 00C             1.309
Water at 1000C           1.31819
Alcohol                  1.329
Water 350C               1.33157
Acetone                  1.36 
Ethyl Alcohol            1.36 
Chlorine                 1.385
Fluorite                 1.434
Opal                     1.450
Quartz                   1.45843
Carbon Tetrachloride     1.460
Plastic                  1.460
Turpentine               1.472
Glycerine                1.473
Plexiglass               1.50
Benzene                  1.501
Glass                    1.51714
Ruby                     1.760
Sapphire                 1.760
Sulphur                  1.960
Crystal                  2.00
Diamond                  2.417
Steel                    2.50
Silicon                  4.24

The index of refraction of a material is actually the ratio of the speed of light in vacuum over the speed of light inside the material. Since the speed of light is the reciprocal of the square root of the material's permittivity times its permeability, then the index of refraction is also given by the square root of the materials's relative permittivity times its relative permeability. Refraction index can be complex when considering the material's absorption. Birefringent materials have two refraction indices, one for each incoming polarization. These two indices are named ordinary index and extraordinary index. Following the iconal equation, one can see the index of refraction as the modulus of the optical path gradient at any given point of a medium.

Because of it's highly ionized state, the state of matter called plasma has a negative index of refraction. In 1968, the Russian physicist Victor Veselago found that in order for a material to have a negative index of refraction, it must behave as a perfect plasma. The index of refraction in plasma is affected by the plasma density and magnetic field. The field causes faraday rotation, which is a change in orientation of the right and left hand polarized vectors of an electromagnetic wave.

Sheldon Schultz, David Smith, and Richard Shelby at the University of California at San Diego constructed a device consisting of copper rings connected by wires that displayed a negative index of refraction in 2000.