Question

# An object and a screen are mounted on an optical bench and a converging lens is placed between then so that a sharp image is obtained on the screen. The linear magnification of the image is 25. The lens is now moved 30 cm towards the screen and a sharp image is again formed on the screen. Find the focal length of the lens.

A
14.0 cm
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B
14.3 cm
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C
14.6 cm
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D
14.9 cm
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Solution

## The correct option is B 14.3 cm Cose - I 1f=1v−1−u magnification =25=vu⇒[v=25u]−(1)1f=125u+1u⇒uf=2625⇒u=2625case−21f=1v−30−1−(u+30)1f=125u−30+1u+30(By (1) ) substitute (2)1f=125(2625)f−30+126f25+301f=126f−30+2526f+750 solving we get f=14.3 cmSo option (B) is correct

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