$A$ wave front is the locus of all the points where the particles of the medium vibrate with the same:

The wavefront of a distant source of unknown shape is approximately

$A$ light wave travels in a vacuum along the $x$-axis. Which of the following may represent the wave front?

The wave theory of light was given by

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.

Explain the reflection of a plane wave using Huygens' principle.