To explain the propagation of light through ether, Huygens proposed the following principles for the construction of a wavefront at a subsequent time from the known position of the wavefront at any given instance.
- Every point on a primary wavefront may be considered as the secondary source of disturbance.
- Secondary waves or wavelets spread out from each one of these secondary source into the medium with the same velocity as the original wave.
- The envelop of all these secondary wavelets (in the forward direction) after any given interval of time gives rise to the secondary wavefront.
Huygens principle is illustrated in the above figure. The XY is the original wavefront. Let v be the velocity of propagation. According to Huygens principle every point 1,2,3,…. etc in XY should be looked upon as the new sources of secondary wavelets. After an interval of time t, the secondary waves travel a distance vt.
With the points 1,2,3,…. etc as the center let us draw spheres of radius vt. The surface X1Y1which is the trace of envelops of all these wavelets in the forward direction is the new wavefront. If the medium is homogeneous, then the velocity is not constant at every point and the appropriate velocity must be used for various wavelets.