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Question

An object(O) is placed at a distance 20 cm from a convex mirror. If a plane mirror is also kept at a distance of 12 cm from the same object as shown, image(I) formed by plane mirror is at the same point at which image is produced by the convex mirror. The focal length of the convex mirror is


A
5 cm
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B
6 cm
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C
7 cm
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D
8 cm
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Solution

The correct option is A 5 cm
For plane mirror,
Object distance = Image distance
Thus, plane mirror will form image 12 cm behind itself.


Since, convex mirror and plane mirror forms the images at the same point.
So, v=+(12+1220)=+4 cm
u=20 cm
From mirror formula,
1v+1u=1f
14120=1f
1f=15
f=5 cm

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