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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 cm
μg=1.5

μw=4/3

fa= focal length of lens in air =8cm

To find fw focal length of lens in water

Sol

we know that

1fa=(μgμa1)(1R11R2)

18=(1.51)(1R11R2)

(1R11R2)=14 ...(1)

When lens is immersed into water,

1fw=(μgμw1)(1R11R2)

1fw=(1.54/31)(1R11R2) ...(2)

From eqn (1) & (2)

1fw=(3×32×41)(14)

=(9×8)8×14

fw=32cm

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