<P> For visible light most transparent media have refractive indices between 1 and 2 . A few examples are given in the adjacent table . These values are measured at the yellow doublet D - line of sodium, 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 . Moissanite lies at the other end of the range with a refractive index as high as 2.65 . 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 . </P> <P> For infrared light refractive indices can be considerably higher . Germanium is transparent in the wavelength region from 2 to 14 μm and has a refractive index of about 4 . A type of new materials, called topological insulator, was recently found holding higher refractive index of up to 6 in near to mid infrared frequency range . Moreover, topological insulator material are transparent when they have nanoscale thickness . These excellent properties make them a type of significant materials for infrared optics . </P> <P> According 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 lower than 1 . 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 frequencies, for absorbing media, in plasmas, and for X-rays . In the X-ray regime the refractive indices are lower than but very close to 1 (exceptions close to some resonance frequencies). As an example, water has a refractive index of 6999999999740000000 ♠ 0.999 999 74 = 1 − 6993260000000000000 ♠ 2.6 × 10 for X-ray radiation at a photon energy of 6985480652946099999 ♠ 30 keV (0.04 nm wavelength). </P> <P> An example of a plasma with an index of refraction less than unity is Earth's ionosphere . Since the refractive index of the ionosphere (a plasma), is less than unity, electromagnetic waves propagating through the plasma are bent "away from the normal" (see Geometric optics) allowing the radio wave to be refracted back toward earth, thus enabling long - distance radio communications . See also Radio Propagation and Skywave . </P>

Can a refractive index be less than 1