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When a ray of light passes from one transparent medium to another,it bends from its path at the surface of separation.This phenomenon is known as 'refraction of light'.

Light travels with different speeds in different media.The bending or refraction of light occurs due to the change in the speed of light as it passes from one medium to another medium.Using Snell's law of refraction,we can write 

                                                  n = sin i/sin r = c/v

As the speed of light(c) in vacuum is greater than the speed of light(v) in any medium or c > v,so sin i > sin r or < i > < r.That is why, a ray of light bends towards the normal as it passes from a rarer medium(air) to a denser medium(glass).