Wednesday, May 23, 2012

What is Index of Refraction?

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.





Many materials have a well-characterized refractive index, but these indices depend strongly upon the frequency of light. Standard refractive index measurements are taken at yellow doublet sodium D line, with a wavelength of 589 nanometres.

There are also weaker dependencies on temperature, pressure/stress, et cetera, as well on precise material compositions (presence of dopants et cetera); for many materials and typical conditions, however, these variations are at the percent level or less. Thus, it is especially important to cite the source for an index measurement if precision is required.

In general, an index of refraction is a complex number with both a real and imaginary part, where the latter indicates the strength of absorption loss at a particular wavelength—thus, the imaginary part is sometimes called the extinction coefficient . Such losses become particularly significant, for example, in metals at short (e.g. visible) wavelengths, and must be included in any description of the refractive index.
Some representative refractive indices
Materialλ (nm)n
Vacuum1 (per definition)
Air at STP1.000277
Gases at 0 °C and 1 atm
Air589.291.000293
Carbon dioxide589.291.00045
Helium589.291.000036
Hydrogen589.291.000132
Liquids at 20 °C
Arsenic trisulfide and sulfur in methylene iodide1.9
Benzene589.291.501
Carbon disulfide589.291.628
Carbon tetrachloride589.291.461
Ethyl alcohol (ethanol)589.291.361
Silicone oil1.52045
Water589.291.3330
Solids at room temperature
Titanium dioxide (also called Titania or Rutile )589.292.496
Diamond589.292.419
Strontium titanate589.292.41
Amber589.291.55
Fused silica (also called Fused Quartz)589.291.458
Sodium chloride589.291.544
Other materials
Liquid helium1.025
Water ice1.31
Cornea (human)1.373/1.380/1.401
Lens (human)1.386 - 1.406
Acetone1.36
Ethanol1.36
Glycerol1.4729
Bromine1.661
Teflon1.35 - 1.38
Teflon AF1.315
Cytop1.34
Sylgard 1841.43
Acrylic glass1.490 - 1.492
Polycarbonate1.584 - 1.586
PMMA1.4893 - 1.4899
PETg1.57
PET1.5750
Crown glass (pure)1.50 - 1.54
Flint glass (pure)1.60 - 1.62
Crown glass (impure)1.485 - 1.755
Flint glass (impure)1.523 - 1.925
Pyrex (a borosilicate glass)1.470
Cryolite1.338
Rock salt1.516
Sapphire1.762–1.778
Sugar Solution, 25%1.3723
Sugar Solution, 50%1.4200
Sugar Solution, 75%1.4774
Cubic zirconia2.15 - 2.18
Potassium Niobate (KNbO3)2.28
Moissanite2.65 - 2.69
Cinnabar (Mercury sulfide)3.02
Gallium(III) phosphide3.5
Gallium(III) arsenide3.927
Zinc Oxide3902.4
Germanium4.01
Silicon5903.96
The refractive index of a material is the most important property of any optical system that uses refraction. It is used to calculate the focusing power of lenses, and the dispersive power of prisms. It can also be used as a useful tool to differentiate between different types of gemstone, due to the unique chatoyance each individual stone displays.

Since refractive index is a fundamental physical property of a substance, it is often used to identify a particular substance, confirm its purity, or measure its concentration. Refractive index is used to measure solids (glasses and gemstones), liquids, and gases. Most commonly it is used to measure the concentration of a solute in an aqueous solution. A refractometer is the instrument used to measure refractive index. For a solution of sugar, the refractive index can be used to determine the sugar content.

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