Q. Four lenses are made from the same type of glass. The radius of curvature of each face is given below. What will have the greatest positive power
A) 10 $cm$ convex and 15 $cm$ concave
B) 5 $cm$ convex and 10 $cm$ concave
C) 15 $cm$ convex and plane
D) 20 $cm$ convex and 30 $cm$ concave
Explanation
(b) Applying the lens maker’s formula
$\frac{1}{f} = P = (n - 1)\left( \frac{1}{R_{1}} - \frac{1}{R_{2}} \right)$
We know that the greater positive power is for that material for which $\frac{1}{R_{1}} - \frac{1}{R_{2}}$ is maximum and positive. For this condition $R_{1}$ and $R_{2}$ should be small as possible but still it must be positive, therefore we must select the combination which has less radius of curvature for convex lens. Hence, option (b) is correct
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.