7(c) a plane wave is incident on
a concave mirror and on reflection we have a spherical wave converging
to the focal point F In a similar manner, we can understand refraction
and reflection by concave lenses and convex mirrors From the above discussion it follows that the total time taken from a
point on the object to the corresponding point on the image is the same
measured along any ray For example, when a convex lens focusses light
to form a real image, although the ray going through the centre traverses
a shorter path, but because of the slower speed in glass, the time taken
is the same as for rays travelling near the edge of the lens