Explain the use of wavefront to understand wave propagation.

Principle: Every point or particle of a wavefront behaves as an independent secondary source and emits secondary spherical waves by itself. After a very small time interval,the surface tangential to all such secondary spherical wavelets gives the position and shape of the new wavefront.
Basically,the Huygens' principle is a geometric construction.
Suppose that $F_{1} F_{2}$ represents a part of a spherical wavefront at $t=0$,which is a wave propagating outwards.
According to Huygens' principle,all points of this wavefront $(F_{1} F_{2})$ (e.g.,$A, B, C, \ldots$) behave as secondary sources. If the velocity of the wave is $v$,then the distance covered in time $\tau$ is $v \tau$.
To determine the shape of the wavefront at $t=\tau$,draw spheres of radius $v \tau$ from each point on the spherical wavefront and draw a common tangent to all these spheres. The surface tangent to these spheres at time $\tau$ gives the position and shape of the new wavefront,which is $G_{1} G_{2}$ in the forward direction. This is a spherical wavefront with center $O$. $A$ backward spherical wavefront $D_{1} D_{2}$ is also formed. The points $A^{\prime}, B^{\prime}, C^{\prime}$ on $G_{1} G_{2}$ act as new secondary sources.