Huygen's principle states that

Explanation

Assumptions on which Huygens. principle is based are:
(i) The medium is homogeneous.
(ii) The medium is isotropic.
Huygens. principle
(a) Each point of a wavelength becomes a source of new disturbance called the secondary wavelets which travel in all directions with the same speed provided the medium remains the same.
(b) The secondary wavefront is the tangent plane joining all the wavefronts.
Fig. (a) represents the trace of secondary wavefront at a certain instant and fig. (b) represents the trace when the light has travelled a great distance.
Rays of light are perpendicular to the wavefront.