Define the term wave front. State Huygen's principle. Consider a plane wave front incident on a thin convex lens. Draw a proper diagram to show how the incident wave front traverses through the lens and after refraction focuses on the focal point of the lens, giving the shape of the emergent wave front.

A wave front is the locus of all the points in space that reach a particular distance by a propagating wave in same phase at any instant.

Huygen's principle is based on two assumptions :

(a) Each point of the wavefront behaves like a source of secondary wavelets. These secondary wavelets spread out in all directions with the same speed as that of the original wave.

(b) When we draw an envelope in the forward direction of the secondary wavelets at any instant, then this envelope tells the new position of the wavefront at that instant.