A beam of light converges to a point P. A lens is placed in the path of the convergent beam 12 cm from P. At what point does the beam converge if the lens is (a) a convex lens of focal length 20 cm. (b) a concave lens of focal length 16 cm.

Explanation

Here the object is virtual and the image is real. `u = +12cm` (object on right, virtual)
(a) `f = +20cm`. Image is real and at 7.5 cm from the lens on its right side.
(b) `f = -16 cm`. Image is real and at 48cm from the lens on its right side.