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In optics the refractive index or index of refraction ''n'' of an material is a dimensionless number that describes how light, propagates through that medium. It is defined as : where ''c'' is the speed of light in vacuum and ''v'' is the speed of light in the medium. For example, the refractive index of water is 1.33, meaning that light travels 1.33 times faster in a vacuum than it does in water The refractive index determines how much light is bent, or refracted, when entering a material. This is the first documented use of refractive indices and is described by Snell's law of refraction, ''n''1 sin''θ''1 = ''n''2 sin''θ''2, where ''θ''1 and ''θ''2 are the angles of incidence and refraction, respectively, of a ray crossing the interface between two media with refractive indices ''n''1 and ''n''2. The refractive indices also determine the amount of light that is reflected when reaching the interface, as well as the critical angle for total internal reflection and Brewster's angle. The refractive index 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 ''v'' = ''c''/''n'', and similarly the wavelength in that medium is ''λ'' = ''λ''0/''n'', where ''λ''0 is the wavelength of that light in vacuum. This implies that vacuum has a refractive index of 1, and that the frequency (''f'' = ''v''/''λ'') of the wave is not affected by the refractive index. The refractive index varies with the wavelength of light. This is called dispersion and causes the splitting of white light into its constituent colors in prisms and rainbows, and chromatic aberration in lenses. Light propagation in absorbing materials can be described using a complex-valued refractive index. The imaginary part then handles the attenuation, while the real part accounts for refraction. The concept of refractive index is widely used within the full electromagnetic spectrum, from X-rays to radio waves. It can also be used with 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. ==Definition== The refractive index ''n'' of an optical medium is defined as the ratio of the speed of light in vacuum, ''c'' = , and the phase velocity ''v'' of light in the medium,〔 : The phase velocity is the speed at which the crests or the phase of the wave moves, which may be different from the group velocity, the speed at which the pulse of light or the envelope of the wave moves. The definition above is sometimes referred to as the absolute refractive index or the absolute index of refraction to distinguish it from definitions where the speed of light in other reference media than vacuum is used.〔 Historically air at a standardized pressure and temperature have been common as a reference medium. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Refractive index」の詳細全文を読む スポンサード リンク
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