{| align=right class="wikitable"

|+Selected refractive indices at λ=589 nm.

For references, see the extended [[List of refractive indices]].

!Material||''n''

|-

|colspan=2 align=center | [[Gas]]es at [[Standard temperature and pressure|0 °C and 1 atm]]

|-

|[[Air]] || {{val|1.000293}}

|-

|[[Helium]] || {{val|1.000036}}

|-

|[[Hydrogen]] || {{val|1.000132}}

|-

|[[Carbon dioxide]] || {{val|1.00045}}

|-

|colspan=2 align=center | [[Liquid]]s at 20 °C

|-

|[[Water]] || 1.333

|-

|[[Ethanol]] || 1.36

|-

|[[Benzene]] || 1.501

|-

|colspan=2 align=center | [[Solid]]s

|-

|[[Ice]] || 1.309

|-

|[[Fused silica]] || 1.46

|-

|[[Poly(methyl methacrylate)|PMMA]] (Plexiglas) || 1.49

|-

|[[Crown glass (optics)|Crown glass]] (typical) || 1.52

|-

|[[Flint glass]] (typical) || 1.62

|-

|[[Diamond]] || 2.42

|-

|}

{{See also|List of refractive indices}}

For [[visible light]] most [[transparency and translucency|transparent]] media have refractive indices between 1 and 2. A few examples are given in the table to the right. These values are measured at the yellow doublet [[sodium]] [[D-line]], with a wavelength of 589 [[nanometers]], as is conventionally done. Gases at atmospheric pressure have refractive indices close to 1 because of their low density. Almost all solids and liquids have refractive indices above 1.3, with [[aerogel]] as the clear exception. Aerogel is a very low density solid that can be produced with refractive index in the range from 1.002 to 1.265.<ref>{{cite paper

| author=Tabata, M. ''et al.''

| title=Development of Silica Aerogel with Any Density

| journal=2005 IEEE Nuclear Science Symposium Conference Record

| year=2005

| url=http://www.ppl.phys.chiba-u.jp/~makoto/publication/N14-191.pdf}}</ref> Diamond lies at the other end of the range with a refractive index as high as 2.42. Most plastics have refractive indices in the range from 1.3 to 1.7, but some [[high-refractive-index polymer]]s can have a value as high as 1.76.<ref>Naoki Sadayori and Yuji Hotta "Polycarbodiimide having high index of refraction and production method thereof" [http://www.google.com/patents?vid=va2WAAAAEBAJ US patent 2004/0158021 A1] (2004)</ref>

For [[infrared]] light refractive indices can be considerably higher. [[Germanium]] is transparent in this region and has a refractive index of about 4, making it an important material for infrared optics.

===Refractive index below 1===

A widespread misconception is that since, according to the [[theory of relativity]], nothing can travel faster than the speed of light in vacuum, the refractive index cannot be lower than 1. This is erroneous since the refractive index measures the [[phase velocity]] of light, which does not carry [[information]]. The phase velocity is the speed at which the crests of the wave move and can be faster than the speed of light in vacuum, and thereby give a refractive index below 1. This can occur close to [[resonance frequency|resonance frequencies]], for absorbing media, in [[plasma (physics)|plasma]]s, and for [[x-ray]]s. In the x-ray regime the refractive indices are lower than but very close to 1 (exceptions close to some resonance frequencies).<ref name=CXRO>{{cite web

|url = http://henke.lbl.gov/optical_constants/

|title = X-Ray Interactions With Matter

|publisher = The Center for X-Ray Optics

|accessdate = 2011-08-30}}</ref>

As an example, water has a refractive index of 1 − {{val|2.6|e=-7}} for X-ray radiation at a photon energy of {{val|30|ul=keV}} (0.04&nbsp;nm wavelength).<ref name=CXRO/>

===Negative refractive index===

{{Main|Negative index metamaterials}}

Recent research has also demonstrated the existence of materials with a negative refractive index, which can occur if [[permittivity]] and [[magnetic permeability|permeability]] have simultaneous negative values. This can be achieved with periodically constructed [[metamaterials]]. The resulting [[negative refraction]] (i.e., a reversal of [[Snell's law]]) offers the possibility of the [[superlens]] and other exotic phenomena.