Explain the refraction of a plane wavefront with a thin convex mirror.

Explanation

Figure shows the plane wavefront XY of a parallel beam of light incident to a thin convex lens and rays coming out through the thin convex lens is shown to be concentrated at second focus point.
To draw a wavefront corresponding to these rays, the circles should be drawn by accepting second focus as a centre. The arc X.Y. of such circles is shown in the figure.
This arc is the wavefront corresponding to the refracted rays at some instant.
Here the distances from A to a and C to care greater then B to b. So the light has taken more distance in the lens going for B to b and the velocity of light in the material of lens is less. Hence, the point b on the wavefront lags behind a and c.