Huygen's principle states that

Explanation

Huygens construction of a plane wavefornt : A plane wavefront may be treated as a part of a spherical or cylindrical wave at a very large distance from a source such that the wavefront has a negligible curvature. Let A, B, C, D, .. be a plane wavefront in a homogeneous isotropic medium in which the speed of light is v.

In a time interval `triangle t,` secondary wavelets with points A, B, C, D, as Secondary sources travel a distance `v triangle t` To find the position of the plane wavefront after a time At, we draw spheres of radii `v trianglet` with A, B, C, D. as centres. The envelope or the surface of tangency to these spheres is a plane ABC and this plane is at a perpendicular distance `v triangle t` from the original wavefront in the direction of the wave. Thus, In an isotropic medium, plane wavefronts are propagated as planes.