# A $16\;cm$ long image of an object is formed by a convex lens on a screen. On moving the lens towards the screen with out changing the positions of object an image of size 9 cm is again formed again on the screen . The size of object is

$(a)\;9\;cm \\ (b)\;12\;cm \\ (c)\;11\;cm \\ (d)\;13\;cm$

$y= \sqrt {y_1 \times y_2}$
$\quad= \sqrt {16 \times 9}$
$\quad= 4 \times 3=12\;cm$
Hence b is the correct answer.