According to Huygen’s Principle:-
Every point of wave front acts as secondary wavelets, which is moving in forward direction.
The sum of the tangents drawn to these wavelets gives the new position of wave front.
Let AB be the wave front at any instant, produced by point source’s’. According to the Huygen’s principle. Every point of wave front acts as the source of secondary wave lets. Then, randomly choose the point 1, 2, 3, 4, 5 in wave front AB. Then these point forms the secondary wavelets having radius ‘ct’ at any instant ‘t’, where ‘c’ is velocity of light. The sum of the tangents drawn into the surface of these secondary wavelets gives the new position of wave front.