Q. Two identical glass ($\mu_g = \frac{3}{2}$) equiconvex lenses of focal length f each are kept in contact. The space between the two lenses is filled with water ($\mu_w = \frac{4}{3}$). The focal length of the combination is
A) $\frac{f}{3}$
B) f
C) $\frac{4f}{3}$
D) $\frac{3f}{4}$
Explanation
[Solution] Each glass lens: R = f. The water forms a concave lens with $\frac{1}{f_w} = \frac{1}{3}\left(-\frac{2}{f}\right)$. Total $\frac{1}{F} = \frac{2}{f} - \frac{2}{3f} = \frac{4}{3f}$, so F = 3f/4. Answer (4).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.