. Huygen’s principle:

(i) All particles on a given wave front act as a new source of disturbance called wavelets. These wavelets travel in all directions with the velocity of a wave.

(ii) The .forward (tangetial) envelope of all these wavelets at any time gives new wave front.

We take a plane wavefront AB incident on a plane surface XX,’ separating the two media.

As soon as the incident wavefront strikes the surface of ‘A’, a wavelet is generated, which spreads out in medium 2. In the same time, the part of wavefront travels a velocity {{V}_{1}} from B to C.

During this duration ‘t‘, the wavelet from A travels in a denser medium with a speed {{V}_{2}} ({{V}_{2}} < {{V}_{1}}) and BC travels a distance AD = {{V}_{2}} x t = {{V}_{2}} x BC/{{V}_{1}}

AD<BC

To determine the shape of the refracted wavefront, we draw a sphere of radius AD, and a tangential plane drawn on a sphere from point C give the refracted wavefront.