Question

A lens is placed between a source of light and a wall. It forms images of area $ A _1 $ and $ A_2 $ on the wall, for its two different positions. The area of the source of light is

• A. $\frac{A_1-A_2}{2}$
• B. $ \frac{1}{A_1} - \frac{1}{A_2}$
• C. $\sqrt{A_1 A_2}$
• D. $\frac{A_1+A_2}{2}$

Answer 1

Answer: (C)

By displacement method, size of object
$(O) = \sqrt{I_1 \times I_2}$
Therefore area of source of light (A) = $\sqrt{A_1 A_2}$