Refractive Index

refractive index

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 (tiny fraction more than 1)
  Water             1.33
  Glass             1.45-1.48
  Lithium niobate   2.25
  Gallium arsenide  3.35
  Silicon           3.5
  Germanium         4.0

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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refractive index [ri′frak·tiv ‚in‚deks]

(optics)

index of refraction

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Refractive Index 

The relative refractive index of two media is the dimensionless ratio n₂₁ of the velocity of the propagation of light (less frequently, of radio waves) in the first medium (v₁) to that in the second medium (v₂); thus, n₂₁ = v₁/v₂. At the same time, the relative refractive index is the ratio of the sines of the angle of incidence a and the angle of refraction β at the interface between the two media: n₂₁ = sin α/sin β. If the first medium is a vacuum (in which the speed of light is c ≃ 3 × 10¹⁰ cm/sec), the refractive index of the medium relative to the vacuum is called the absolute refractive index: n = c/v. The relative refractive index is the ratio between the absolute refractive indexes of the two media: n₂₁ = n₂/n₁.

The refractive index depends on the wavelength λ (frequency v) of the radiation (seeDISPERSION OF LIGHT). The absolute refractive index is related to the dielectric constant ∊ and magnetic permeability μ of the medium by the equation nλ = (∊ and μ are also functions of λ). The absolute refractive index of a medium is determined by the polarizability of its component particles, as well as by the structure of the medium and its state of aggregation (seeCLAUSIUS-MOSSOTTI EQUATION, LORENTZ-LORENZ FORMULA, AND ). In the case of mediums characterized by optical anisotropy, whether natural or induced, the refractive index depends on the radiation’s direction of propagation and its polarization. Many crystals are typical anisotropic media (seeCRYSTAL OPTICS). Media that absorb radiation are described by a complex refractive index ñ = n(1 + ik), where the term containing only n usually corresponds to the transmission of light without scattering and κ = kλ/4π describes absorption (k is the absorption coefficient of the medium). (See.)

In the range of visible light, the refractive index varies from 1.3 to 4.0 for weakly absorbing transparent solids. For liquids it ranges from 1.2 to 1.9, and for gases under normal conditions from 1.000035 (He) to 1.000702 (Xe).