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Question

# An object is placed at a distance of 30 cm from the lens. Its image is formed at a distance of 10 cm on the same side of the lens. Find the focal length of the lens used.

A
f=10 cm
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B
f=10 cm
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C
f=+15 cm
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D
f=15 cm
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Solution

## The correct option is D f=−15 cmGiven: Image distance v=−10 cm Object distance u=−30 cm (Using the sign conventions) Let focal length of the lens be f. Using the lens formula, 1f=1v−1u 1f=(1−10)−(1−30) f=−15 cm Here, negative sign for focal length shows that the lens is concave.

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