The refractive index (or index of refraction) of a medium is a measure of how much the velocity of a wave is reduced inside that medium. For example, typical sodalime glass has a refractive index close to 1.5, which means that in glass, light travels at 1 / 1.5 = 2/3 the speed of light in a vacuum. Two common properties of glass and other transparent materials are directly related to their refractive index. First, light rays change direction when they cross the interface from air to the material, an effect that is used in lenses. Second, light reflects partially from surfaces that have a refractive index different from that of their surroundings.
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The refractive index, n, of a medium is defined as the ratio of the velocity, c, of a wave phenomenon such as light or sound in a reference medium to the phase velocity, v_{p} in the medium itself:
It is most commonly used in the context of light with vacuum as a reference medium, although historically other reference media (e.g. air at a standardized pressure and temperature) have been common. It is usually given the symbol n. In the case of light, it equals
where ε_{r} is the material's relative permittivity, and μ_{r} is its relative permeability. For most materials, μ_{r} is very close to 1 at optical frequencies, therefore n is approximately . Contrary to a widespread misconception, n may be less than 1, for example for Xrays.^{[1]} This has practical technical applications, such as effective mirrors for Xrays based on total external reflection. Another example is that the n of electromagnetic waves in plasmas is less than 1.
The phase velocity is defined as the rate at which the crests of the waveform propagate; that is, the rate at which the phase of the waveform is moving. The group velocity is the rate at which the envelope of the waveform is propagating; that is, the rate of variation of the amplitude of the waveform. Provided the waveform is not distorted significantly during propagation, it is the group velocity that represents the rate at which information (and energy) may be transmitted by the wave, for example the velocity at which a pulse of light travels down an optical fiber.
A closely related quantity is refractivity, which in atmospheric applications is denoted N and defined as N = 10^{6}(n  1) the 10^{6} factor is used because for air, n deviates from unity at most a few parts per thousand.
The speed of all electromagnetic radiation in vacuum is the same, approximately 3×10^{8} m/s, and is denoted by c. Therefore, if v is the phase velocity of radiation of a specific frequency in a specific material, the refractive index is given by
or inversely
This number is typically greater than one: the higher the index of the material, the more the light is slowed down (see also Cherenkov radiation). However, at certain frequencies (e.g. near absorption resonances, and for Xrays), n will actually be smaller than one. This does not contradict the theory of relativity, which holds that no informationcarrying signal can ever propagate faster than c, because the phase velocity is not the same as the group velocity or the signal velocity.
Sometimes, a "group velocity refractive index", usually called the group index is defined:
where v_{g} is the group velocity. This value should not be confused with n, which is always defined with respect to the phase velocity. The group index can be written in terms of the wavelength dependence of the refractive index as
where λ is the wavelength in vacuum. At the microscale, an electromagnetic wave's phase velocity is slowed in a material because the electric field creates a disturbance in the charges of each atom (primarily the electrons) proportional to the permittivity of the medium. The charges will, in general, oscillate slightly out of phase with respect to the driving electric field. The charges thus radiate their own electromagnetic wave that is at the same frequency but with a phase delay. The macroscopic sum of all such contributions in the material is a wave with the same frequency but shorter wavelength than the original, leading to a slowing of the wave's phase velocity. Most of the radiation from oscillating material charges will modify the incoming wave, changing its velocity. However, some net energy will be radiated in other directions (see scattering).
If the refractive indices of two materials are known for a given frequency, then one can compute the angle by which radiation of that frequency will be refracted as it moves from the first into the second material from Snell's law.
If in a given region the values of refractive indices n or n_{g} were found to differ from unity (whether homogeneously, or isotropically, or not), then this region was distinct from vacuum in the above sense for lacking Poincaré symmetry.
Recent research has also demonstrated the existence of negative refractive index, which can occur if the real parts of both permitivity ε_{eff} and permeability μ_{eff} have simultaneous negative values, although such is a necessary but not sufficient condition. Not thought to occur naturally, this can be achieved with periodically constructed Negative index metamaterials. The resulting negative refractive index (i.e. a reversal of Snell's law) offers the possibility of the superlens, metamaterial cloaking device and other exotic phenomena.^{[2]}^{[3]}
In real materials, the polarization does not respond instantaneously to an applied field. This causes dielectric loss, which can be expressed by a permittivity that is both complex and frequency dependent. Real materials are not perfect insulators either, i.e. they have nonzero direct current conductivity. Taking both aspects into consideration, we can define a complex index of refraction:
Here, n is the refractive index indicating the phase velocity as above, while κ is called the extinction coefficient, which indicates the amount of absorption loss when the electromagnetic wave propagates through the material. (See Mathematical descriptions of opacity.) Both n and κ are dependent on the frequency (wavelength). Note that the sign of the complex part is a matter of convention, which is important due to possible confusion between loss and gain. The notation above, which is usually used by physicists, corresponds to waves with time evolution given by e ^{− iωt}.
The effect that n varies with frequency (except in vacuum, where all frequencies travel at the same speed, c) is known as dispersion, and it is what causes a prism to divide white light into its constituent spectral colors, explains rainbows, and is the cause of chromatic aberration in lenses. In regions of the spectrum where the material does not absorb, the real part of the refractive index tends to increase with frequency. Near absorption peaks, the curve of the refractive index is a complex form given by the Kramers–Kronig relations, and can decrease with frequency.
Since the refractive index of a material varies with the frequency (and thus wavelength) of light, it is usual to specify the corresponding vacuum wavelength at which the refractive index is measured. Typically, this is done at various welldefined spectral emission lines; for example, n_{D} is the refractive index at the Fraunhofer "D" line, the centre of the yellow sodium double emission at 589.29 nm wavelength.
The Sellmeier equation is an empirical formula that works well in describing dispersion, and Sellmeier coefficients are often quoted instead of the refractive index in tables. For some representative refractive indices at different wavelengths, see list of indices of refraction.
As shown above, dielectric loss and nonzero DC conductivity in materials cause absorption. Good dielectric materials such as glass have extremely low DC conductivity, and at low frequencies the dielectric loss is also negligible, resulting in almost no absorption (κ ≈ 0). However, at higher frequencies (such as visible light), dielectric loss may increase absorption significantly, reducing the material's transparency to these frequencies.
The real and imaginary parts of the complex refractive index are related through use of the Kramers–Kronig relations. For example, one can determine a material's full complex refractive index as a function of wavelength from an absorption spectrum of the material.
The dielectric constant (which is often dependent on wavelength) is simply the square of the (complex) refractive index in a nonmagnetic medium (one with a relative permeability of unity). The refractive index is used for optics in Fresnel equations and Snell's law; while the dielectric constant is used in Maxwell's equations and electronics.
Where , , , n, and are functions of wavelength:
Conversion between refractive index and dielectric constant is done by:
The refractive index of certain media may be different depending on the polarization and direction of propagation of the light through the medium. This is known as birefringence or anisotropy and is described by the field of crystal optics. In the most general case, the dielectric constant is a rank2 tensor (a 3 by 3 matrix), which cannot simply be described by refractive indices except for polarizations along principal axes.
In magnetooptic (gyromagnetic) and optically active materials, the principal axes are complex (corresponding to elliptical polarizations), and the dielectric tensor is complexHermitian (for lossless media); such materials break timereversal symmetry and are used e.g. to construct Faraday isolators.
In crystalline calcium carbonate (calcite), the birefringent (uniaxial) optics depend on directional differences in the structure. The index of refraction also depends on composition, and can be calculated using the Gladstone–Dale relation.
The strong electric field of high intensity light (such as output of a laser) may cause a medium's refractive index to vary as the light passes through it, giving rise to nonlinear optics. If the index varies quadratically with the field (linearly with the intensity), it is called the optical Kerr effect and causes phenomena such as selffocusing and selfphase modulation. If the index varies linearly with the field (which is only possible in materials that do not possess inversion symmetry), it is known as the Pockels effect.
If the refractive index of a medium is not constant, but varies gradually with position, the material is known as a gradientindex medium and is described by gradient index optics. Light traveling through such a medium can be bent or focused, and this effect can be exploited to produce lenses, some optical fibers and other devices. Some common mirages are caused by a spatiallyvarying refractive index of air.
In general, the refractive index of a glass increases with its density. However, there does not exist an overall linear relation between the refractive index and the density for all silicate and borosilicate glasses. A relatively high refractive index and low density can be obtained with glasses containing light metal oxides such as Li_{2}O and MgO, while the opposite trend is observed with glasses containing PbO and BaO as seen in the diagram at the right.
In 1908, Hermann Minkowski calculated the momentum of a refracted ray, p, where E is energy of the photon, c is the speed of light in vacuum and n is the refractive index of the medium as follows:^{[6]}
In 1909, Max Abraham proposed the following formula for this calculation:^{[7]}
Rudolf Peierls raises this inconsistency in More Surprises in Theoretical Physics.^{[8]} Ulf Leonhardt, Chair in Theoretical Physics at the University of St Andrews, has discussed this problem, including experiments to resolve it.^{[9]}
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.
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 (see Brix).
In GPS, the index of refraction is utilized in raytracing to account for the radio propagation delay due to the Earth's electrically neutral atmosphere.
See also: Abraham–Minkowski controversy and discussion on that page.
