Editing Refractive index (section)

{{Short description|Property in optics}}
[[File:Refraction photo.png|thumb|A [[ray (optics)|ray]] of light being [[refraction|refracted]] through a glass slab|alt=refer to caption]]
[[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 air or vacuum to the speed in the medium. 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''<sub>1</sub> sin ''θ''<sub>1</sub> = ''n''<sub>2</sub> sin ''θ''<sub>2</sub>}}, where {{math|''θ''<sub>1</sub>}} and {{math|''θ''<sub>2</sub>}} 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''<sub>1</sub>}} and {{math|''n''<sub>2</sub>}}. 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''<sub>0</sub>/''n''}}, where {{math|''λ''<sub>0</sub>}} 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.

The refractive index may vary with wavelength. This causes white light to split into constituent colors when refracted. This is called [[dispersion (optics)|dispersion]]. This effect can be observed in [[Prism (optics)|prisms]] and [[rainbow]]s, and as [[chromatic aberration]] in lenses. Light propagation in [[Absorption (electromagnetic radiation)|absorbing]] materials can be described using a [[complex number|complex]]-valued refractive index.<ref name="Attwood">{{cite book|title=Soft X-rays and extreme ultraviolet radiation: principles and applications|author=Attwood, David |page=60|isbn=978-0-521-02997-1|year=1999|publisher=Cambridge University Press }}</ref> The [[Imaginary number|imaginary]] part then handles the [[attenuation]], while the [[Real number|real]] part accounts for refraction. For most materials the refractive index changes with wavelength by several percent across the visible spectrum. Consequently, refractive indices for materials reported using a single value for {{mvar|n}} must specify the wavelength used in the measurement.

The concept of refractive index applies across the full [[electromagnetic spectrum]], from [[X-ray]]s to [[radio wave]]s. It can also be applied to [[wave]] phenomena such as [[sound]]. In this case, the [[speed of sound]] is used instead of that of light, and a reference medium other than vacuum must be chosen.<ref name=Kinsler>{{cite book | last = Kinsler | first = Lawrence E. | title = Fundamentals of Acoustics | url = https://archive.org/details/fundamentalsacou00kins_265 | url-access = limited | publisher = John Wiley | year = 2000 | isbn = 978-0-471-84789-2 | page = [https://archive.org/details/fundamentalsacou00kins_265/page/n151 136]}}</ref> Refraction also occurs in oceans when light passes into the [[halocline]] where salinity has impacted the density of the water column.

For [[lens]]es (such as [[eye glasses]]), a lens made from a high refractive index material will be thinner, and hence lighter, than a conventional lens with a lower refractive index.  Such lenses are generally more expensive to manufacture than conventional ones.