State and explain Huygen's principle.

Explanation

Huygen.s principle is used to find the position of the given wavefront at any instant of time, provided its present location is known.
According to this principle
(i) Every point on the given wavefronts acts as a source of new disturbance, callled secondary wavelets which travels in all directions. The velocity of secondary wavelets is same as that of primary wavefront and is equal to velocity of light.
(ii) A tangent to the secondary wavelets in the forward direction at any instant gives the position of secondar wavefront at that instant.

(iii) In a homogeneous medium, the wavefront is always normal to the direction of propagation of waves.
Consider a primary wave front AB and taken points 1, 2, 3, 4, ... etc. on this wavefront. The distance travellled by light in .t. seconds `=cxxt`, where c is the velocity of light.
Now taking each point 1, 2, 3, 4, ... as centre and radius equal to `cxxt`, draw spheres. These spheres are called secondary wavelts. Now draw a tangent or envelop `A_(2)B_(2)` to these spheres. The surface `A_(2)B_(2)` is the position of secondary wavefront after t seconds.