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A point object O is placed at a distance o 20cm from a convex lens of focal 10 cm as shown. At what distance x from lens should a concave mirror of focal length 60 cm can be placed so that final image coincides with object.

$(a)\;20\;cm \\ (b)\;30\;cm \\ (c)\;40\;cm \\ (d)\;50\;cm $

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Object is placed at a distance of 2 f from lens (f= focal length of lens)
ie the image formed by the lens will be at a distance of 2 f or 20 cm from lens.
So, if concave mirror is placed in this position , the first image will be formed at its pole and it will reflect all the rays symmetrically to other side as shown and the final image will coincide with the object.
Hence a is the correct answer.
answered Feb 19, 2014 by meena.p

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