Which of the following generates a plane wavefront?

What is the most important point of Huygens' wave theory?

Light waves propagate rectilinearly due to:

Write the limitation of Huygen's principle.

State Huygens' principle. What is the fundamental principle on which it is based?

What is the shape of the wavefront in each of the following cases:
$(a)$ Light diverging from a point source.
$(b)$ Light emerging out of a convex lens when a point source is placed at its focus.
$(c)$ The portion of the wavefront of light from a distant star intercepted by the Earth.