index of refraction

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The ratio of the speed of light in a vacuum versus the speed of light through another medium. It is always greater than one, unless in a perfect vacuum, as the presence of matter retards the speed of light. When passing through a medium, long wavelengths refract less than short wavelengths, so the refractive index of a medium depends on the wavelength of light passed through it.

A property of a material that changes the speed of light, computed as the ratio of the speed of light in a vacuum to the speed of light through the material. When light travels at an angle between two different materials, their refractive indices determine the angle of transmission (refraction) of the light beam. In general, the refractive index varies based on the frequency of the light as well, thus different colors of light travel at different speeds. High intensities also can change the refractive index.

The refractive index of a vacuum is 1.0, and air is a tiny fraction greater than 1.0. The higher the index, the slower the speed of light through the medium, because the speed through the material is the speed of light (c) over the refractive index (n), thus speed = c/n. Following are common refractive indices. See fiber optics glossary.

                    Refractive
  Material          Index (n)

  Vacuum            1.0

  Air               1.0**

  Water             1.33

  Glass             1.45-1.48

  Lithium niobate   2.25

  Gallium arsenide  3.35

  Silicon           3.5

  Germanium         4.0

 ** = air is a tiny franction
       greater than 1.0

Refractive Indices

When light travels at an angle between two materials, light bends according to their refractive indices. In order to reflect, light must be on the wider side of the critical angle.

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A measurement of a substance's ability to bend incidental light as the light passes through it. Instruments called refractometers measure refractive indices. These refractometers can come in handheld units or as instruments that resemble microscopes. When the liquid is added to the measuring device, a clear line of demarcation forms. This line develops when light passes through the substance. The line typically shows a darker area on one side and a lighter area on the other. This line is then read against a hairline scale within the instrument that delineates the refractive index scale (or Brix scale in the other case). Handheld units are typically less accurate in the measurement of refractive index, and are more effective in measuring soluble solids or Brix than refractive index. Although some of the more expensive handheld models can now do both. The degree of Brix is often used to test syrups and juices. See Optical Rotation, Brix.

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abbr.: RI; symbol: n; for a given medium, a measure of the degree of refraction induced by the medium when light or other electromagnetic radiation passes through it. The refractive index of a medium is the ratio of the speed of light in a vacuum, c₀, to the speed of light in the medium, c; i.e. n = c₀/c. For many practical purposes the speed of light in air may be used instead of that in a vacuum.

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For a list of words related to index of refraction, see:

• Optics - index of refraction: number expressing speed of light in given medium, as ratio of either speed of light in a vacuum or speed in some specified medium to that in the given medium

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