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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

1. √(A_1 A_2)
2. (A_1 + A_2)/2
3. A_1 A_2/2
4. 1/(A_1 + A_2)

Correct answer: √(A_1 A_2)

Solution

The lens formula and the concept of magnification imply that the product of the areas of the images formed in two positions equals the square of the area of the source. Thus, the area of the source is the geometric mean of A_1 and A_2.

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