Its most elementary occurrence (and historically the first one) is in Snell's law of refraction, n1sinθ1= n2sinθ2, where θ1 and θ2 are the angles of incidence of a ray crossing the interface between two media with refractive indices n1 and n2. Brewster's angle, the critical angle for total internal reflection, and the reflectivity of a surface also depend on the refractive index, as described by the Fresnel equations.
A measure of the extent to which a substance slows down
light waves passing through it. The index of refraction of a substance is equal
to the ratio of the velocity of light in a vacuum to its speed in that
substance. Its value determines the extent to which light is refracted when
entering or leaving the substance.
Some representative refractive indices
Some representative refractive indices
| Material | Index |
| Vacuum | 1.00000 |
| Air at STP | 1.00029 |
| Ice | 1.31 |
| Water at 20 C | 1.33 |
| Acetone | 1.36 |
| Ethyl alcohol | 1.36 |
| Sugar solution(30%) | 1.38 |
| Fluorite | 1.433 |
| Fused quartz | 1.46 |
| Glycerine | 1.473 |
| Sugar solution (80%) | 1.49 |
| Typical crown glass | 1.52 |
| Crown glasses | 1.52-1.62 |
| Spectacle crown, C-1 | 1.523 |
| Sodium chloride | 1.54 |
| Polystyrene | 1.55-1.59 |
| Carbon disulfide | 1.63 |
| Flint glasses | 1.57-1.75 |
| Heavy flint glass | 1.65 |
| Extra dense flint, EDF-3 | 1.7200 |
| Methylene iodide | 1.74 |
| Sapphire | 1.77 |
| Rare earth flint | 1.7-1.84 |
| Lanthanum flint | 1.82-1.98 |
| Arsenic trisulfide glass | 2.04 |
| Diamond | 2.417 |
Recent research has also demonstrated the existence of the negative refractive index, which can occur if permittivity and permeability have simultaneous negative values. This can be achieved with periodically constructed metamaterials. The resulting negative refraction (i.e., a reversal of Snell's law) offers the possibility of the superlens and other exotic phenomena.
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