Chapter-10|Wave Optics|NCERT 12th Physics:
Huygens’ principle gives the shape of the wavefront at any time and allows us to determine the shape of the wavefront at a latter time.
According to Huygens’ Principle
(I) Every point on a wavefront acts as source of secondary disturbances which emits their own wavelets called secondary wavelets. Secondary wavelets spread in all directions with speed of wave.
(II) The common tangent on secondary wavelets in forward direction gives new wavefront.