First slide

The lens

Question

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

Moderate

A

15
B

18
C

24
D

30

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) $\frac{1}{- 24} + \frac{1}{d} = \frac{1}{40}$
$\Rightarrow d = 15 c m$

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