A convex lens of focal length 40 cm is held coaxially above mirror of focal length 18 cm. A luminous point object placed d cm above the lens on its axis gives rise to final image coincident with itself. Then d is equal to
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a
15
b
18
c
24
d
30
answer is A.
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Detailed Solution
The lens forms the image on the centre of curvature of mirror, at a distance of 36 cm from the mirror. so that the rays will retrace their path. i.e, υ=−24 (for lens) 1−24+1d=140⇒d=15 cm