As an example, water has a refractive index of 0.999 999 74 = 1 − 2.6 ×10 −7 for X-ray radiation at a photon energy of 30 keV (0.04 nm wavelength). In the X-ray regime the refractive indices are lower than but very close to 1 (exceptions close to some resonance frequencies). This can occur close to resonance frequencies, for absorbing media, in plasmas, and for X-rays. 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. The refractive index measures the phase velocity of light, which does not carry information. Refractive index below unityĪccording to the theory of relativity, no information can travel faster than the speed of light in vacuum, but this does not mean that the refractive index cannot be less than 1. These properties are potentially important for applications in infrared optics. Moreover, topological insulators are transparent when they have nanoscale thickness. A type of new materials termed "topological insulators", was recently found which have high refractive index of up to 6 in the near to mid infrared frequency range. Germanium is transparent in the wavelength region from 2 to 14 µm and has a refractive index of about 4. įor infrared light refractive indices can be considerably higher. Most plastics have refractive indices in the range from 1.3 to 1.7, but some high-refractive-index polymers can have values as high as 1.76. Moissanite lies at the other end of the range with a refractive index as high as 2.65. Aerogel is a very low density solid that can be produced with refractive index in the range from 1.002 to 1.265. Almost all solids and liquids have refractive indices above 1.3, with aerogel as the clear exception. Gases at atmospheric pressure have refractive indices close to 1 because of their low density. These values are measured at the yellow doublet D-line of sodium, with a wavelength of 589 nanometers, as is conventionally done. A few examples are given in the adjacent table. PMMA (acrylic, plexiglas, lucite, perspex)įor visible light most transparent media have refractive indices between 1 and 2. For references, see the extended List of refractive indices. Where the coefficients A and B are determined specifically for this form of the equation.
![sopra refractive index database sopra refractive index database](https://www.ftgsoftware.com/images/crn1.png)
Ratio of the speed of light in vacuum to that in the medium A ray of light being refracted in a plastic block