A 16 cm long image of an object is formed by a convex lens on a screen placed normal to its principal axis. On moving the lens towards the screen, without changing the positions of the object and the screen, a 9 cm long image is formed again on the screen. The size of the object is:

12 cm

$y=\sqrt{y_1 \times y_2}$

$=\sqrt{16 \times 9}$

= 4 × 3 = 12 cm.