State and explain Huygens' principle.

(N/A) Principle: Every point on a wavefront acts as an independent source of secondary spherical wavelets,which spread out in all directions with the speed of light in that medium. The new wavefront at any later time is the forward envelope (tangential surface) of these secondary wavelets.
Explanation:
$1$. Huygens' principle is a geometric method to determine the shape of a wavefront at a future time if its current shape is known.
$2$. Consider a spherical wavefront $F_1 F_2$ at time $t=0$ originating from a point source $O$.
$3$. According to the principle,every point on $F_1 F_2$ (such as $A, B, C, \dots$) acts as a secondary source. If the wave speed is $v$,then in a time interval $\tau$,each secondary wavelet travels a distance $v\tau$.
$4$. To find the new wavefront at time $t = \tau$,draw spheres of radius $v\tau$ centered at each point on the original wavefront. The forward common tangent surface $G_1 G_2$ to these spheres represents the new wavefront.
$5$. The backward tangent surface $D_1 D_2$ is also formed,but it is generally ignored as the wave propagates in the forward direction. The points $A', B', C'$ on $G_1 G_2$ then act as new secondary sources for further propagation.