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Q.

The image of a square hole in a screen illuminated by light is obtained on another screen with the help of converging lens. The distance of the hole from the lens is 40 cm. If the area of the image is nine times that of the hole, the focal length of the lens is

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a

60 cm

b

50 cm

c

30 cm

d

75 cm

answer is A.

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Detailed Solution

Areal magnification $\frac{v^{2}}{u^{2}} = 9 \Rightarrow v = 3 u; f = \frac{u v}{u + v}$

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The image of a square hole in a screen illuminated by light is obtained on another screen with the help of converging lens. The distance of the hole from the lens is 40 cm. If the area of the image is nine times that of the hole, the focal length of the lens is