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Refraction of Light : Happens in Transparent medium when a light travels from one medium to another, refraction takes place.A ray of light bends as it moves from one medium to another . Refraction is due to change in the speed of light as it enters from one transparent medium to another.
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Refraction is the phenomenon of the bending of light when it passes from one medium into another medium (transparent to light).

The amount of the light ray bending is dependent on the optical density of the medium and the angle of incidence with respect to the normal drawn at the point of incidence on the surface separating  the two media.

The Snell's law tells us that the Sine of angle of incidence  divided by sine of angle of refraction is the refractive index of the medium 2 with respect to the medium 1.

   Sin i / Sin r  = Mu2 / Mu1

The refraction is due to difference in optical densities of materials and different  velocities of light in different media.  Light bends in order to reach a point in medium 2 at the minimum time.

See diagrams.
Sir, what do you mean by optical density?
it is not same as the mass / volume .. it is in terms of the optical conduction / resistance the media offers to the light..
it is related to proportion of the intensity of light transmitted through it