Question

In the displacement method, the sizes of image in the two positions of the lens between the object and the screen are $9\text{cm and}4\text{cm}$ respectively. The size of the object must be:

Answer 1

If $l_1$ and $l_2$ are the lengths of the images in the two positions of the lens, between the object and the screen, then the length of the object is given by
$l=\sqrt{l_1{l}_2}=\sqrt{9\times 4}=6\text{cm}$

Hence, Option $\left(C\right)$ is the correct answer.