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Focal length of a convex lens of refractive index 1.5 is 2 cm. Focal length of the lens when immersed in a liquid of refractive index of 1.25 will be

1. 10 cm
2. 2.5 cm
3. 5 cm
4. 7.5 cm

Correct answer: 5 cm

Solution

The focal length of a lens in a medium is given by the lens maker's formula. When immersed in a liquid, the effective refractive index changes to (1.5/1.25). Using the formula, the new focal length is calculated as 5 cm.

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