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Question

# A container is filled with water (μ=1.33) upto a height of 33.25cm. A concave mirror is placed 15cm above the water level and the image of an object placed at the bottom is formed 25cm below the water level. The focal length of the mirror is (nearly) :

A
10 cm
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B
15 cm
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C
19 cm
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D
25 cm
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Solution

## The correct option is C 19 cmOne important thing to note in this question is that even after reflection from the mirror, there will be refraction. So, the image formation without water would have taken place at the apparent depth of the real image below the level of water.So, for image formation, (by sign convention)−u=15+ apparent depth of object−v=15+ apparent depth of imageApparent depth of object= 33.251.33=25cmu=−40cmApparent depth of image= 251.33=18.8cmv=−33.8cm1f=133.8+140From this we get f≈19cm

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