Consider a point at the focal point of a convergent lens. Another convergent lens of short focal length is placed on the other side. What is the nature of the wavefronts emerging from the final image ?

Here final image is point like which is formed at position I on the common axis of both the lenses. As this point lies on wavefront, it behaves like self independent secondary source and emits secondary spherical waves in all the directions. At a given instant of time, if we think of a surface, circumscribing to secondary spherical waves (i.e. a surface tangential from outside) then it gives us shape and position of new wavefront at that time. In the present case its shape is found to be spherical.