# What is refractive index?

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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, vp in the medium itself:

   n = \frac{c}{v_{\mathrm {p}}}.


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

   n=\sqrt{\epsilon_r\mu_r},

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DefinThe 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, vp in the medium itself:
   n = \frac{c}{v_{\mathrm {p}}}.


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

   n=\sqrt{\epsilon_r\mu_r},


where εr is the material's relative permittivity, and μr is its relative permeability. For most materials, μr is very close to 1 at optical frequencies, therefore n is approximately \sqrt{\epsilon_r}. Contrary to a widespread misconception, n may be less than 1, for example for X-rays.[1] This has practical technical applications, such as effective mirrors for X-rays based on total external reflection. Another example is that the n of electromagnetic waves in plasmas is less than 1.TDefinitionsThe 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, vp in the medium itself:

   n = \frac{c}{v_{\mathrm {p}}}.


It is most commonly used in the context of light with vacuum asindexof