When a point source of light is placed at the focus of a convex lens,light emerges from the lens. What is the shape of the wavefront of the emerging light?

State and explain Huygens' principle.

The wavefront of a distant source of unknown shape is approximately

Derive the laws of refraction using the concept of Huygens' principle of wavefronts.

For light diverging from a point source,

$A$ light beam travelling along the $X$-axis with a planar wavefront is incident on a medium of thickness $t$. In the region where light is falling,the refractive index varies such that $(dn/dy) > 0$. The light beam on the other side of the medium will emerge: