Q. Two thin lenses, one of focal length $+ 60\ cm$ and the other of focal length $- 20\ cm$ are put in contact. The combined focal length is
A) $+ \ 15\ cm$
B) $- 15cm$
C) $+ \ 30\ cm$
D) $- 30\ cm$
Explanation
(d) $\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} = \frac{1}{60} + \frac{1}{( - 20)} \Rightarrow F = - 30$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.