Explain the refraction of a plane wavefront by a thin convex lens.

(N/A) The figure shows a plane wavefront $XY$ of a parallel beam of light incident on a thin convex lens. The rays emerging from the lens are concentrated at the second focus point $F$.
To draw the refracted wavefront corresponding to these rays,we draw an arc of a circle centered at the focus $F$. The arc $X^{\prime}Y^{\prime}$ shown in the figure represents this wavefront at a specific instant.
In this process,the path length traveled by the light ray passing through the center of the lens (point $B$ to $b$) is greater than the path length traveled by the rays passing through the edges (points $A$ to $a$ and $C$ to $c$). Since the speed of light in the lens material is less than in air,the central part of the wavefront is delayed more than the edges. Consequently,the point $b$ on the wavefront lags behind points $a$ and $c$,resulting in a spherical converging wavefront.