Many materials have a well-characterized refractive index, but these indices often depend strongly upon the frequency of light, causing optical dispersion. Standard refractive index measurements are taken at the "yellow doublet" sodium D line, with a wavelength (λ) of 589 nanometers.
There are also weaker dependencies on temperature, pressure/stress, etc., as well on precise material compositions (presence of dopants, etc.); for many materials and typical conditions, however, these variations are at the percent level or less. Thus, it's 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.
Name of material | λ (nm) | Refractive index no. n | Reference |
---|---|---|---|
Vacuum | 1 (by definition) | ||
Air at STP | 1.000273 | [ citation needed ] | |
Gases at 0 °C and 1 atm | |||
Air | 589.29 | 1.000293 | [1] |
Carbon dioxide | 589.29 | 1.00045 | [2] [3] [4] |
Helium | 589.29 | 1.000036 | [1] |
Hydrogen | 589.29 | 1.000132 | [1] |
Liquids at 20 °C | |||
Arsenic trisulfide and sulfur in methylene iodide | 1.9 | [5] | |
Carbon disulfide | 589.29 | 1.628 | [1] |
Benzene | 589.29 | 1.501 | [1] |
Carbon tetrachloride | 589.29 | 1.461 | [1] |
Silicone oil (nD25) | 589.29 | 1.393–1.403 | [6] |
Kerosene | 1.39 | ||
Ethanol (ethyl alcohol) | 589.29 | 1.361 | [1] |
Acetone | 1.36 | ||
Water | 589.29 | 1.333 | [1] |
10% glucose solution in water | 589.29 | 1.3477 | [7] |
20% glucose solution in water | 589.29 | 1.3635 | [7] |
60% glucose solution in water | 589.29 | 1.4394 | [7] |
Solids at room temperature | |||
Silicon carbide (moissanite; 6H form) | 589.29 | 2.65 | [8] |
Titanium dioxide (rutile phase) | 589.29 | 2.614 | [9] [10] |
Diamond | 589.29 | 2.417 | [1] |
Strontium titanate | 589.29 | 2.41 | [11] |
Tantalum pentoxide | 589.29 | 2.15 | [12] |
Amber | 589.29 | 1.55 | [1] |
Sodium chloride | 589.29 | 1.544 | [13] |
Fused silica (a pure form of glass, also called fused quartz) | 589.29 | 1.458 | [1] [14] |
Other materials | |||
Liquid helium | 1.025 | ||
Perfluorohexane (Fluorinert FC-72) | 1.251 | [15] | |
Water ice | 1.31 | ||
TFE/PDD (Teflon AF) | 1.315 | [16] [17] | |
Cryolite | 1.338 | ||
Cytop | 1.34 | [18] | |
Polytetrafluoroethylene (Teflon) | 1.35–1.38 | [19] | |
Sugar solution, 25% | 1.3723 | [20] | |
Cornea (human) | 1.373/1.380/1.401 | [21] | |
Lens (human) | 1.386–1.406 | ||
Liver (human) | 964 | 1.369 | [22] |
Intestinal mucosa (human) | 964 | 1.329–1.338 | [23] |
Ethylene tetrafluoroethylene (ETFE) | 1.403 | [24] | |
Sylgard 184 (polydimethylsiloxane) | 1.4118 | [25] | |
Sugar solution, 50% | 1.4200 | [20] | |
Polylactic acid | 1.46 | [26] | |
Pyrex (a borosilicate glass) | 1.470 | [27] | |
Vegetable oil | 1.47 | [28] | |
Glycerol | 1.4729 | ||
Sugar solution, 75% | 1.4774 | [20] | |
Poly(methyl methacrylate) (PMMA) | 1.4893–1.4899 | ||
Halite (rock salt) | 1.516 | ||
Plate glass (window glass) | 1.52 | ||
Crown glass (pure) | 1.50–1.54 | ||
PETg | 1.57 | ||
Polyethylene terephthalate (PET) | 1.5750 | ||
Polycarbonate | 150 | 1.60 | [30] |
Crown glass (impure) | 1.485–1.755 | ||
Flint glass (pure) | 1.60–1.62 | ||
Bromine | 1.661 | ||
Flint glass (impure) | 1.523–1.925 | ||
Sapphire | 1.762–1.778 | ||
Boron nitride | 2–2.14 | [31] | |
Cubic zirconia | 2.15–2.18 | [32] | |
Potassium niobate (KNbO3) | 2.28 | ||
Zinc oxide | 390 | 2.4 | |
Cinnabar (mercury sulfide) | 3.02 | Birefringent: nω = 2.905 nε = 3.256 [33] | |
Silicon | 1200 - 8500 | 3.42–3.48 | [34] |
Gallium(III) phosphide | 3.5 | ||
Gallium(III) arsenide | 3.927 | ||
Germanium | 3000 - 16000 | 4.05–4.1 | [35] |
Glass is a non-crystalline solid that is often transparent, brittle and chemically inert. It has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics.
In optics, the refractive index of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Circular birefringence and circular dichroism are the manifestations of optical activity. Optical activity occurs only in chiral materials, those lacking microscopic mirror symmetry. Unlike other sources of birefringence which alter a beam's state of polarization, optical activity can be observed in fluids. This can include gases or solutions of chiral molecules such as sugars, molecules with helical secondary structure such as some proteins, and also chiral liquid crystals. It can also be observed in chiral solids such as certain crystals with a rotation between adjacent crystal planes or metamaterials.
The Sellmeier equation is an empirical relationship between refractive index and wavelength for a particular transparent medium. The equation is used to determine the dispersion of light in the medium.
Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are described as birefringent or birefractive. The birefringence is often quantified as the maximum difference between refractive indices exhibited by the material. Crystals with non-cubic crystal structures are often birefringent, as are plastics under mechanical stress.
Fused quartz, fused silica or quartz glass is a glass consisting of almost pure silica (silicon dioxide, SiO2) in amorphous (non-crystalline) form. This differs from all other commercial glasses in which other ingredients are added which change the glasses' optical and physical properties, such as lowering the melt temperature. Fused quartz, therefore, has high working and melting temperatures, making it less desirable for most common applications.
The Faraday effect or Faraday rotation, sometimes referred to as the magneto-optic Faraday effect (MOFE), is a physical magneto-optical phenomenon. The Faraday effect causes a polarization rotation which is proportional to the projection of the magnetic field along the direction of the light propagation. Formally, it is a special case of gyroelectromagnetism obtained when the dielectric permittivity tensor is diagonal. This effect occurs in most optically transparent dielectric materials under the influence of magnetic fields.
A metamaterial is any material engineered to have a property that is rarely observed in naturally occurring materials. They are made from assemblies of multiple elements fashioned from composite materials such as metals and plastics. These materials are usually arranged in repeating patterns, at scales that are smaller than the wavelengths of the phenomena they influence. Metamaterials derive their properties not from the properties of the base materials, but from their newly designed structures. Their precise shape, geometry, size, orientation and arrangement gives them their smart properties capable of manipulating electromagnetic waves: by blocking, absorbing, enhancing, or bending waves, to achieve benefits that go beyond what is possible with conventional materials.
In optics, an ARROW is a type of waveguide that uses the principle of thin-film interference to guide light with low loss. It is formed from an anti-resonant Fabry–Pérot reflector. The optical mode is leaky, but relatively low-loss propagation can be achieved by making the Fabry–Pérot reflector of sufficiently high quality or small size.
An antireflective, antiglare or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses, other optical elements, and photovoltaic cells to reduce reflection. In typical imaging systems, this improves the efficiency since less light is lost due to reflection. In complex systems such as cameras, binoculars, telescopes, and microscopes the reduction in reflections also improves the contrast of the image by elimination of stray light. This is especially important in planetary astronomy. In other applications, the primary benefit is the elimination of the reflection itself, such as a coating on eyeglass lenses that makes the eyes of the wearer more visible to others, or a coating to reduce the glint from a covert viewer's binoculars or telescopic sight.
In optics, Cauchy's transmission equation is an empirical relationship between the refractive index and wavelength of light for a particular transparent material. It is named for the mathematician Augustin-Louis Cauchy, who defined it in 1837.
Arsenic triselenide is an inorganic chemical compound with the chemical formula As2Se3.
Germanium dioxide, also called germanium(IV) oxide, germania, and salt of germanium, is an inorganic compound with the chemical formula GeO2. It is the main commercial source of germanium. It also forms as a passivation layer on pure germanium in contact with atmospheric oxygen.
Athermalization, in the field of optics, is the process of achieving optothermal stability in optomechanical systems. This is done by minimizing variations in optical performance over a range of temperatures.
The optical properties of all liquid and solid materials change as a function of the wavelength of light used to measure them. This change as a function of wavelength is called the dispersion of the optical properties. The graph created by plotting the optical property of interest by the wavelength at which it is measured is called a dispersion curve.
Negative-index metamaterial or negative-index material (NIM) is a metamaterial whose refractive index for an electromagnetic wave has a negative value over some frequency range.
A high-refractive-index polymer (HRIP) is a polymer that has a refractive index greater than 1.50.
A plasmonic metamaterial is a metamaterial that uses surface plasmons to achieve optical properties not seen in nature. Plasmons are produced from the interaction of light with metal-dielectric materials. Under specific conditions, the incident light couples with the surface plasmons to create self-sustaining, propagating electromagnetic waves known as surface plasmon polaritons (SPPs). Once launched, the SPPs ripple along the metal-dielectric interface. Compared with the incident light, the SPPs can be much shorter in wavelength.
A. R. Forouhi and I. Bloomer deduced dispersion equations for the refractive index, n, and extinction coefficient, k, which were published in 1986 and 1988. The 1986 publication relates to amorphous materials, while the 1988 publication relates to crystalline. Subsequently, in 1991, their work was included as a chapter in “The Handbook of Optical Constants”. The Forouhi–Bloomer dispersion equations describe how photons of varying energies interact with thin films. When used with a spectroscopic reflectometry tool, the Forouhi–Bloomer dispersion equations specify n and k for amorphous and crystalline materials as a function of photon energy E. Values of n and k as a function of photon energy, E, are referred to as the spectra of n and k, which can also be expressed as functions of the wavelength of light, λ, since E = HC/λ. The symbol h represents Planck’s constant and c, the speed of light in vacuum. Together, n and k are often referred to as the “optical constants” of a material.
The Forouhi–Bloomer model is a mathematical formula for the frequency dependence of the complex-valued refractive index. The model can be used to fit the refractive index of amorphous and crystalline semiconductor and dielectric materials at energies near and greater than their optical band gap. The dispersion relation bears the names of Rahim Forouhi and Iris Bloomer, who created the model and interpreted the physical significance of its parameters. The model is aphysical due to its incorrect asymptotic behavior and non-Hermitian character. These shortcomings inspired modified versions of the model as well as development of the Tauc–Lorentz model.
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