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Question

# A point object is placed at a distance of 18 cm from a convex lens on its principal axis. Its image is formed on the other side of the lens at 27 cm. Calculate the focal length of the lens.

A

10.0 cm

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B

10.8 cm

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C

12.0 cm

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D

15.0 cm

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Solution

## The correct option is B 10.8 cm According to the sign convention, the object is placed on the left side of the lens, therefore object-distance is negative,i.e. u = - 18 cm. Since the image is formed on the other side, the image-distance is positive,i.e., n = + 27 cm. Using len's formula, 1v - 1u = 1f, we have 1+27 - 1−18 = 1f f = 10.8 cm

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