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Question

If an object is placed at a distance of 30 cm and the image is formed at a distance of 10 cm on the same side of the lens, then find the focal length of the lens used.

A
10 cm
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B
15 cm
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C
20 cm
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D
25 cm
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Solution

The correct option is B 15 cmGiven: Image distance, v=−10 cm Object distance, u=−30 cm [Using the sign conventions, as both object and image are on the left side of the lens] Let focal length be f. Using the lens formula: 1v−1u=1f 1−10−1−30=1f f=−15 cm Here negative sign shows that the lens is concave.

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