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Refraction is the bending of light when it travels from one medium to another with a different refractive index. The law of refraction, also known as Snell's law, governs how light is refracted when it passes through a boundary between two transparent materials. The law states that the ratio of the sines of the angle of incidence (I) to the angle of refraction (r) is a constant value, which is equal to the ratio of the refractive indices (n1 and n2) of the two media:
n1 sin I = n2 sin r
where n1 and n2 are the refractive indices of the two media, and I and r are the angles of incidence and refraction, respectively, measured with respect to the normal to the surface separating the two media.