Huygens's Principle

A wave theory of light based on the concept of wavelets or secondary waves spreading from each point affected by a disturbance and conspiring to give a fresh wavefront which envelops the wavelets which create it. The amplitude in a secondary wavelet falls off in proportion to ( 1 + Cosθ ), where θ is the angle with the forward direction. Huygens conceived the waves as longitudinal in nature and could not explain polarization.
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         Knowing the shape and location of a wavefront at any instant t, Huygen's principle enables us to determine the shape and location of the new wavefront at a later time t + ∆t. This principle consists of two parts: 

 Every point of a wavefront may be considered as a source of secondary wavelets which spread out in forward direction with a speed equal to the speed of propagation of the wave.

The new position of the wavefront after a certain interval of time can be found by constructing a surface that touches all the secondary wavelets.

     The principle is illustrated in Fig 1 given below. AB represent the wavefront at any instant t. To determine the wavefront at time t + ∆t, draw secondary wavelets with center at various points on the wavefront AB and radius as c∆t where c is the speed of the propagation of the waves as shown in Fig 1. The new wavefront at time t + ∆t is A'B' which is a tangent envelope to all the secondary wavelets. Fig 2 shows a similar construction for a plane wavefront.

  Fig 1 . Spherical Wavefront                      Fig 2 . Plane Wavefront