Huygen's principle : Huygen's principle states that every point of a wave front behaves as the source of small secondary wavelets which spread out in all directions with a velocity equal to the velocity of light. The new wave front is then found by constructing a surface tangential to the secondary wavelets.

Consider a point source of light ‘S'. It sends out spherical wave fronts around it. Let AB represents a spherical wave front at any instant. According to Huygens principle each point on this AB acts as a source of light called secondary source. 

The light emerges from these secondary sources again as small spheres called wave lets. The radius of these spheres is given by Ct. where ‘C' is the velocity of light. A surface tangential to all these wave lets gives an envelope CD. This CD gives the new position of the wavefront AB after a time interval ‘t'. This process of construction of new wavefronts at different positions can be continued. Thus light advances in a direction perpendicular to the wavefront. This method of construction of wavefronts is the essence of Huygen's principle (hypothesis)