The wavelength λ of light in a material is determined by the refractive index according to λ = λ0 / n, where λ0 is the wavelength of the light in vacuum. Brewster's angle, the critical angle for total internal reflection, and the reflectivity of a surface is also affected by the refractive index. These material parameters can be calculated using the Fresnel equations.[4]
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 other than light, e.g. 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.[5] .."
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Physicists refer to the law of refraction as Snell's law, despite the other independent discoveries.
Snell's Law
The angle through which light is refracted when it passes between two media depends on a property of each medium known as the index of refraction or refractive index. This angle also depends on the angle at which the light strikes the surface between the media.
Snell's law is a mathematical relationship between the refractive indices of the two media and the sines of the angles the light makes when travels in each media. The equation for Snell's law when light travels from medium 1 to medium 2 is:
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Where n1 and n2 are the indices of refraction of each medium. Theta1 and theta2 are the angles the light makes when it strikes the surface between the media and when it leaves the surface, the incident and refracted angles. Physicists do not measure these angles from the surface between the two media. Rather physicists measure the angles from a line that is drawn perpendicular to the surface, which is called the normal. In mathematics the word normal means perpendicular.