The wavefront of a wave has what direction with respect to the wave motion?

$A$ point source of light is placed at the focus of a concave mirror. Consider only paraxial rays. The shapes of the wavefronts of incident and reflected lights respectively are

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

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

Explain the refraction of a plane wave by a thin prism.

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.