Explain the reflection of a plane wavefront from a concave mirror.

(N/A) When a plane wavefront is incident on a concave mirror,the rays parallel to the principal axis reflect and converge towards the focal point $F$.
The incident plane wavefront $XY$ and the reflected spherical wavefront $X'Y'$ are shown in the figure.
As the rays travel towards the mirror,the central ray hits the pole $O$ of the mirror,while the marginal rays hit the edges. Since the central ray travels a longer distance to reach the mirror and then reflects back,it covers a different path length compared to the marginal rays.
Specifically,the point $b$ on the reflected wavefront,which corresponds to the central ray,is delayed relative to points $a$ and $c$ on the reflected wavefront. This is because the central ray has to travel to the pole $O$ and back,whereas the marginal rays reflect from the edges. Consequently,the reflected wavefront becomes spherical,converging towards the focus $F$.