Q. The radius of the convex surface of plano-convex lens is $20\ cm$ and the refractive index of the material of the lens is 1.5. The focal length of the lens is
A) $30\ cm$
B) $50\ cm$
C) $20\ cm$
D) $40\ cm$
Explanation
(d) $\frac{1}{F} = (1.5 - 1)\left( \frac{1}{20} - \frac{1}{\infty} \right) \Rightarrow F = 40\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.