Q. The radius of curvature for a convex lens is 40 $cm$, for each surface. Its refractive index is 1.5. The focal length will be
A) $40\ cm$
B) $20\ cm$
C) $80\ cm$
D) $30\ cm$
Explanation
(a) By formula $\frac{1}{f} = (\mu - 1)\left( \frac{1}{R_{1}} - \frac{1}{R_{2}} \right)$
$= (1.5 - 1)\left( \frac{1}{40} + \frac{1}{40} \right) = 0.5 \times \frac{1}{20} = \frac{1}{40}$
$\therefore f = 40\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.