Q. The distance between a convex lens and a plane mirror is $10\ cm$. The parallel rays incident on the convex lens after reflection from the mirror form image at the optical centre of the lens. Focal length of lens will be
A) 10 $cm$
B) 20 $cm$
C) 30 $cm$
D) Cannot be determined
Explanation
(b) From the figure,
Using property of plane mirror
Image distance $=$ Object distance
$f - 10 = 10 \Rightarrow f = 20\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.