A convex lens of focal length 40 cm is held co- axially 12 cm above a mirror of focal length 18 cm. An object held x cm above the lens gives rise to an image coincident with it. Then x is equal to:
A)12 cm
B) 15 cm
C)18 cm
D) 30 cm
Correct Answer:
B
Solution :
[b] \[u=24\,cm,\]\[v=x\] using \[\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\] or \[\frac{1}{x}-\frac{1}{24}=\frac{1}{40}\] \[\therefore \] \[x=15\,cm\]