The index of refraction is defined as the speed of light in vacuum divided by the speed of light in the medium. The indices of refraction of some common substances are given below with a more complete description of the indices for optical glasses given elsewhere. The values given are approximate and do not account for the small variation of index with light wavelength which is called dispersion. Material n Material n Vacuum 1.000 Ethyl alcohol 1.362 Air 1.000277 Glycerine 1.473 Water 4/3 Ice 1.31 Carbon disulfide 1.63 Polystyrene 1.59 Methylene iodide 1.74 Crown glass 1.50-1.62 Diamond 2.417 Flint glass 1.57-1.7
Optics for Optometry