With the help of a diagram,explain the Huygens' principle for the propagation of light in a medium.

Explanation

Huygens Principal states that :
(i) Each point on a given wavefront acts as a souce of secondary wavelets, sending out distrubances in all directions in a same way as the original source of light does.
(ii) The new positive of wavefront at any instant is the envelope of the secondary wavelets at that instant.
This principal is used to find the new position of wavefront at a later time.
Explanation. Cosider a point source of light. Let AB be a section of primary spherical wavefront at any time t. In order to find the new position of wavefront at time `t + Delta t`, consider a centres draw spherers each of radius c `Delta t`. These spheres are called secondary wavelets originating from points on primary wavefront. Parts A.B. and A..B. of the envelope are temed as forward secondary wavefront and backward secondary wavefront respectively. It may be pointed out that the effective part of the secondary wavelets is the portion which lies on the forward secondary wavefront. Huygens assumed that the portion of the secondary wavelents, which lies on the backward secondary wavefront, does not exist at all. However, many years later, Voigt and Kirehoff mathematically proved that the contribution. of a wavelet in any direction making angle `theta` with normal to the wavelet is proportional to `(1)/(2) (1 + cos theta)`. Obviously, for the portion of the wavelet, which lies just on the back of the secondary wavefront, `theta` is equal to `180^(@)` and likewise the factor `(1)/(2) (1 + cos theta)` is equal to zero. Therefore, the forward secondary wavefront A.B. is the new protion of the wavefont at time `t + Delta t`.