Huygens principle :
(i) In a wavefront, every particles act as new source of disturbance which are known as wavelets that travel in every directions with velocity of wave
(ii) The forward (tangential) envelope of all these wavelets at any time gives new wavefront.
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 V1 from B to C.
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.
