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     When light travels from one transparent medium into another, a change of direction takes place at the surface of separation. This bending of light is called refraction. The refracted ray bends towards or away from the normal according as the second medium is optically denser or rarer than the first medium. 

 

     Laws of refraction

• The incident ray, the refracted ray and the normal to the surface of separation at the point of incidence lie in the same plane.
• The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a given pair of media and for a given colour of light. This is called Snell's law.

If i is the angle of incidence and r the angle of refraction, then, ^sini/_sinr = constant. This consant is called refractive index of the second medium with respect to the first.

                           

Since the path of light is reversable, for a ray going from medium 2 to medium 1