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Question

An object is placed at a distance of 15 cm from a convex lens of focal length 10 cm. On the other side of the lens, a convex mirror is placed at its focus such that the image formed by the combination coincides with the object itself. The focal length of the convex mirror is :

A
20cm
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B
100cm
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C
25cm
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D
30cm
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Solution

The correct option is B 100cm
For the lens,

1v1u=1f

1v=110115

Thus, v=30cm

Now, for the mirror, this image acts as the object.

u=3010=20cm

Also, v=25cm
Hence,

125+120=1f

f=100cm

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