Introduction. Refractive index, commonly known as the index of refraction, is a measurement of how much a light beam bends when it passes through one medium and into another. The refractive index n is defined as the ratio of the sine of the angle of incidence to the sine of the angle of refraction, i.e., n = sin I / sin r, where I is the angle of incidence of a ray in vacuum (angle between the incoming ray and the perpendicular to the surface of a medium, called the normal) and r is the angle of refraction (angle between the ray in the medium and the normal).

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