Q. A double convex lens of focal length $20\ cm$ is made of glass of refractive index 3/2. When placed completely in water ${(\ }_{a}\mu_{\omega} = 4/3)$, its focal length will be
A) $80\ cm$
B) $15\ cm$
C) $17.7\ cm$
D) $22.5\ cm$
Explanation
(a) $f_{water} = 4 \times f_{air}$ , air lens is made up of glass
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.