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Question

# Final image of a point source O placed at the bottom of the container filled with water in it is formed at 25 cm below the water level as shown in the figure. Focal length of the mirror (in cm) is

A
15
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B
18
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C
25
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D
30
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Solution

## The correct option is B −18Using the concept of apparent depth, Apparent depth =Actual depthμ From question, object distance will be u=15+33.25μ=15+33.251.33 ⇒u=40 cm And image distance will be v=15+25μ=15+251.33 ⇒v=33.80 cm For concave mirror, u=−40 cm; v=−33.80 cm By using mirror formula 1v+1u=1f ⇒1f=1−40+1−33.75 ∴f≈−18 cm Hence, option (b) is correct.

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