Q.Explain the construction of a spherical wavefront by using Huygens' principle.

Huygens' construction for a point source radiating spherical waves.

A point source $S$ emits light uniformly in all directions, so at any instant the locus of points that have been reached by disturbance for the same time forms a sphere centred at $S$ — this is a spherical wavefront.

According to Huygens' principle, every point on this wavefront (of radius $r$ at time $t$) becomes a source of new secondary wavelets, which spread out with the same speed $v$ as the original wave. After a further time interval $\Delta t$, each secondary wavelet has radius $v\Delta t$; the surface tangent to all these secondary wavelets (the "envelope") is the new wavefront, now a sphere of radius $r+v\Delta t$, still centred at $S$. Repeating this construction at successive instants generates the sequence …