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Question

A convex glass lens (μg=1.5) has a focal length of 8 cm when placed in air. What is the focal length lens when it is immersed in water? (μω=43)

A
32 cm
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B
60 cm
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C
16 cm
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D
30 cm
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Solution

The correct option is A 32 cmμg=1.5 μw=4/3fa= focal length of lens in air =8cmTo find →fw→ focal length of lens in waterSolwe know that1fa=(μgμa−1)(1R1−1R2)18=(1.5−1)(1R1−1R2)(1R1−1R2)=14 ...(1)When lens is immersed into water,1fw=(μgμw−1)(1R1−1R2)1fw=(1.54/3−1)(1R1−1R2) ...(2)From eqn (1) & (2)1fw=(3×32×4−1)(14)=(9×−8)8×14fw=32cm

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