What is the significance of Huygens' conception of secondary waves?

The idea of secondary wavelets for the propagation of a wave was first given by

$A$ 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:

Describe the short history of light.

Carefully analyze the given diagrams and choose the correct option regarding the refraction of a plane wavefront by a thin convex lens:

$A$ parallel beam of light strikes a piece of transparent glass having a cross-section as shown in the figure below. The correct shape of the emergent wavefront will be (figures are schematic and not drawn to scale)-