Question

Define wave front. Draw the shape of refracted wave front when the plane incident wave undergoes refraction from optically denser medium to rarer medium. Hence prove Snell’s law of refraction.

Answer 1

A locus of points in space which oscillate in the same phase is called a wave front . 

That is, the phase difference between any two points of the same wave front is zero .

Let us assume an incident wave front AB incident on an interface between 2 media with refraction indices n₁ > n₂.

According to Huygen’s wave theory:

• A source of light  gives out waves in all directions.
• Every point on the wave front is a source of disturbance. These disturbances called secondary wavelets,  travel in all directions.
• The time taken by secondary wavelets to travel from primary to secondary wave front is a constant.

Answer 2

A locus of points, which oscillate in phase is called a wave front.

OR

A wave front is defined as a surface of constant phase.

Diagram

Proof: n₁ sin i = n₂ sin r (Derivation)

This is the Snell’s law of refraction.