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 :

20cm

100cm

25cm

30cm

A

100cm

B

20cm

C

25cm

D

30cm

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Solution

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For the lens,

1v−1u=1f

1v=110−115

Thus, v=30cm

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

u=30−10=20cm

Also, v=−25cm Hence,

−125+120=1f

f=100cm

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