Q. A concave lens with unequal radii a curvature made of glass $(\mu_{g} = 1.5)$ has a focal length of 40 cm. If it is immersed in a liquid of refractive index $\mu_{l}$=2, then
A) It behave like a convex lens of 80 cm focal length
B) It behave like a concave lens of 20 cm focal length
C) Its focal length becomes 60 cm
D) Nothing can be said
Explanation
(a) $- \frac{1}{40} = (1.5 - 1)\left( \frac{1}{R_{1}} - \frac{1}{R_{2}} \right)$
$\frac{1}{R_{1}} - \frac{1}{R_{2}} = - \frac{1}{20}$
Now, $\frac{1}{f} = \left( \frac{1.5}{2} - 1 \right)\left( - \frac{1}{20} \right)$
Or $\ \frac{1}{f} = - \frac{0.5}{2}\left( - \frac{1}{20} \right)$
Or $\ \frac{1}{f} = \frac{1}{80}\ $or$\ f = 80\ cm$
It behave like a convex lens of focal length 80 cm
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.