The speed of a waves changes when it passes from one medium to another and it bends. This bending of a wave when it enters a medium where its speed is different is called Refraction. The bending of a wave depends on refractive index of the medium and the angle between the ray and the line perpendicular (normal) to the surface separating the two media.
The medium with greater refractive index is called the denser medium and the one with smaller refractive index is called the rarer medium. The speed of wave is less in denser medium than that in a rarer medium. When wave travels from denser to rarer medium it bends away from the normal and angle of refraction is greater than angle of incident. When wave travels from rarer to denser medium the refracted ray bends towards the normal and the angle of refraction is less than the angle of incidence.
Snell’s Law
A Dutch physicist, Willebrord Snell derived a law which states that the ratio of sines of angle of incidence (i) and angle of refraction (r) is equivalent to the ratio of velocities (v1 / v2) in the two media, or equivalently, to the opposite ratio of the indices of refraction (n2 / n1):
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