23) is the familiar thin lens formula Though we derived
it for a real image formed by a convex lens, the formula is valid for both
convex as well as concave lenses and for both real and virtual images It is worth mentioning that the two foci, F and F¢, of a double convex
or concave lens are equidistant from the optical centre The focus on the
side of the (original) source of light is called the first focal point, whereas
the other is called the second focal point