Question

A convex lens forms a real image 9 cm 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 4 cm long on the screen. Then the length of the object is

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

Answer 1

Answer: (C)

$6cm$

If the image size be $I_1$ and $I_2$
The object size is given by relations obtained in displacement method.

$OS=\sqrt{I_1{I}_2}$

Thus, OS = $\sqrt{36}$

Or, $OS=6$