Q. In the figure, an air lens of radii of curvature $10\ cm\ (R_{1} = R_{2} = 10\ cm)$ is cut in a cylinder of glass $(\mu = 1.5)$. The focal length and the nature of the lens is
A) 15 $cm$, concave
B) 15 $cm$, convex
C) $\infty$, neither concave nor convex
D) 0, concave
Explanation
(a) $\frac{1}{f} = \left( \ _{g}\mu_{a} - 1 \right)\left( \frac{1}{R_{1}} - \frac{1}{R_{2}} \right) = \left( \frac{2}{3} - 1 \right)\left( \frac{2}{10} \right)$
$\Rightarrow f = - 15\ cm,$ so behaves as concave lens
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.