Q. The refractive index of the material of a double convex lens is 1.5 and its focal length is 5 cm. If the radii of curvature are equal, the value of the radius of curvature (in cm) is
A) 5.0
B) 6.5
C) 8.0
D) 9.5
Explanation
(a) $\frac{1}{f} = (\mu - 1)\left( \frac{1}{R_{1}} - \frac{1}{R_{2}} \right)$
For double convex lens, $R_{1} = R,\ R_{2} = - R$
$\frac{1}{5} = (1.5 - 1)\left( \frac{1}{R} + \frac{1}{R} \right)$
$or\ \frac{1}{5} = 0.5\ \times \frac{2}{R}$
$or\ R = 5\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.