Explain the reflection of plane wavefront from concave mirror. 

These depicts the parallel incident of rays and reflection on the incoming concave mirror and the rays focus near the focal point $\mathrm{F}$.

The wavefront $X Y$ of incident ray and wavefront $X^{\prime} Y^{\prime}$ of reflected ray are also shown in figure.

Here point $b$ corresponding to the reflected rays is left behind relation to $a$ and $c$.

Because the rays at the pole $\mathrm{O}$ of the mirror have taken less distances than the rays reflecting from the edge of the mirror. Thus it is little late reflection occur at $\mathrm{O}$ and hence point $b$ remains behind the other points on the reflected wavefront.