Q. The plano-convex lens of focal length 20 cm and 30 cm are placed together to form a double convex lens. The final focal length will be
A) 12 cm
B) 60 cm
C) 20 cm
D) 30 cm
Explanation
(a) Equivalent focal length
$\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}}$
$\ = \frac{1}{20} + \frac{1}{30}$
$\therefore F = \frac{20 \times 30}{20 + 30}$
$= \frac{600}{50} = 12cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.