Refractive index: Difference between revisions

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[[File:Refraction at interface.svg|thumb|170px|Refraction of a light ray|alt=Illustration of the incidence and refraction angles]]

In [[optics]], the '''refractive index''' (or '''refraction index''') of an [[optical medium]] is the [[ratio]] of the apparent speed of light in the mediumair or vacuum to the speed in airthe or vacuummedium. The refractive index determines how much the path of [[light]] is bent, or [[refraction|refracted]], when entering a material. This is described by [[Snell's law]] of refraction, {{math|1=''n''₁ sin ''θ''₁ = ''n''₂ sin ''θ''₂}}, where {{math|''θ''₁}} and {{math|''θ''₂}} are the [[angle of incidence (optics)|angle of incidence]] and angle of refraction, respectively, of a ray crossing the interface between two media with refractive indices {{math|''n''₁}} and {{math|''n''₂}}. The refractive indices also determine the amount of light that is [[reflectivity|reflected]] when reaching the interface, as well as the critical angle for [[total internal reflection]], their intensity ([[Fresnel equations]]) and [[Brewster's angle]].<ref name="Hecht">{{cite book | author = Hecht, Eugene | title = Optics | publisher = Addison-Wesley | year = 2002 | isbn = 978-0-321-18878-6}}</ref>

The refractive index, <math>n</math>, can be seen as the factor by which the speed and the [[wavelength]] of the radiation are reduced with respect to their vacuum values: the speed of light in a medium is {{math|1=''v'' = c/''n''}}, and similarly the wavelength in that medium is {{math|1=''λ'' = ''λ''₀/''n''}}, where {{math|''λ''₀}} is the wavelength of that light in vacuum. This implies that vacuum has a refractive index of 1, and assumes that the [[frequency]] ({{math|1=''f'' = ''v''/''λ''}}) of the wave is not affected by the refractive index.