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The ratio of the speed of light in vaccum to the speed of light in the medium is termed as '' refractive index '' of medium.
Let us consider the medium I, which is optically denser than medium 2.
Let the speed of light be
in medium I and 
in medium II.
Always, note that
A plane wave AB propagates and hits the interface at an angle i. and can be the time taken be the wavefront to travel the distance BC.
Now, we want to draw the refracted wavefront.
We can draw a sphere of radius
with A as centre. Let the surface tangent to the sphere passing through point C, as the refracted wavefront.
Now,
Let the surface be tangent to the sphere at E.
In
In
On dividing both the equations, we finally have,
Hence,
This is the verified snell's law.