__Huygens principle :__ 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 *X*_{1}Y_{1}
which 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.

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