Write Huygen's principle and explain it. 

Principle : Every point or particle of a wavefront behave as an independent secondary source and emits by itself secondary spherical waves. After a very small time interval the surface tangential to all such secondary spherical wavelets give the position and shape of new wavefront.

Basically the Huygen's principle is a geometric construction.

Suppose, that $\mathrm{F}_{1} \mathrm{~F}_{2}$ represents a part of spherical wavefront at $t=0$ which is a wave propagating outwards.

According to Huygen's principle all points of this wavefront $\left(\mathrm{F}_{1} \mathrm{~F}_{2}\right)(\mathrm{A}, \mathrm{B}, \mathrm{C}, \ldots)$ behave as secondary sources and velocity of wave is $v$, then distance covered in time $\tau$ is $v \tau$.

To determine the shape of 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 sphere then at time $t$ after $\tau$ time gives the position and shape of new wavefront which is $\mathrm{G}_{1} \mathrm{G}_{2}$ in the forward direction. This is a spherical wavefront with centre $\mathrm{O}$ and $\mathrm{D}_{1} \mathrm{D}_{2}$ spherical wavefront is found backward. The points $\mathrm{A}^{\prime}, \mathrm{B}^{\prime}, \mathrm{C}^{\prime}$ on $\mathrm{G}_{1} \mathrm{G}_{2}$ act as secondary source.