Explain the refraction of plane wavefront of light through a convex lens.

Explanation

Let AB represent a plane wavefront. The secondary wavelets from A and B reach the points A' and B' respectively in same the time as the secondary wavelets reach from `P_1 and P_2` . Thus the points `A', P_2 and B'` are in the same phase. This explains why a parallel beam of lights on refraction through a convex lens gets converged and gives a spherical wavefront, whereas the plane wavefront diverges in the case of concave lens.