Q. Focal length of a convex lens of refractive index 1.5 in $2\ cm$. Focal length of lens when immersed in a liquid of refractive index of 1.25 will be
A) $10\ cm$
B) $2.5\ cm$
C) $5\ cm$
D) $7.5\ cm$
Explanation
(c) $\frac{f_{l}}{f_{a}} = \frac{\ _{a}\mu_{g} - 1}{\ _{l}\mu_{g} - 1} = \frac{\ _{a}\mu_{g} - 1}{\frac{\ _{a}\mu_{g}}{\ _{a}\mu_{l}} - 1} \Rightarrow \frac{f_{1}}{2} = \frac{1.5 - 1}{\frac{1.5}{1.25} - 1} \Rightarrow f_{1} = 5cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.