question_answer

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  \[\frac{1}{x}-\frac{1}{24}=\frac{1}{40}\] \[\therefore \] \[x=15\,cm\]