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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=1v1u magnification =25=vu[v=25u](1)1f=125u+1uuf=2625u=2625case2
1f=1v301(u+30)1f=125u30+1u+30(By (1) ) substitute (2)1f=125(2625)f30+126f25+301f=126f30+2526f+750 solving we get f=14.3 cm
So option (B) is correct

2006091_1015177_ans_cb6e67fa58914dcabc9de2bf37b5889f.png

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