Write Huygen's principle and explain it.

Explanation

Principle : Every point or particle of a wavefror behave as an independent secondary source an emits by itself secondary spherical waves. After very small time interval the surface tangential t all such secondary spherical wavelets give th position and shape of new wavefront.
Basically the Huygen.s principle is a geometri construction.
Suppose, that `F_(1),F_(2)` represents a part of spherica wavefront at t = 0 which is a wave propagating outwards.

According to Huygen.s principle all points of this wavefront `(F_(1)F_(2))` (A, B, C, ...) behave as secondary sources and velocity of wave is v, then distance covered in time `tua` is `vtau`.
To determine the shape of wavefront at `t=tau`, draw spheres of radius `vtau` 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 `G_(1)G_(2)` in the forward direction. This is a spherical wavefront with centre O and `D_(1)D_(2)` spherical wavefront is found backward. The points A., B., C. on `G_(1)G_(2)` act as secondary source.