Question

A convex lens forms a real image $9cm$ long on a screen. Without altering the position of the object and the screen, the lens is displaced and we get again a real image $4cm$ long on the screen. Then the length of the object is-

• A. $9cm$
• B. $4cm$
• C. $6cm$
• D. $36cm$

Answer 1

The correct option is C $6cm$

For the two positions of the convex lens, when the object and the screen are unaltered, let, $I$ and $I\prime$ be the sizes of the images.

It is given, that $I=9cm$ and $I\prime =4cm$.

We know, object size, $O=\sqrt{II\prime }$

$\therefore$ The length of the object is $=\sqrt{9\times 4}cm=\sqrt{36}cm=6cm$

So, the correct answer is - (C) $6cm$