A convex lens of focal length 40cm is held at a distance 12cm coaxially above a concave mirror of focale length 18cm.

Q. A luminous point object placed d cm above the lens on its axis gives rise to an imag coincident iwth itself, then the value of d is equal to

Options

A. 12 cm
B. 15 cm, convex
C. 18 cm
D. 30 cm

Answer

B

Explanation

The image by lens should be formed at centre of curvature of mirror which is `(36-12)=24cm` above lens.
Image by lens:
`u=-x,v=-24,f=40`
`(1)/(v)-(1)/(u)=(1)/(f)implies(1)/(-24)-(1)/(-x)=(1)/(40)`
`(1)/(x)=(1)/(40)+(1)/(24)=(3+5)/(120)impliesx=15cm`