Q. Two thin lenses of focal length 20 cm and 25 cm are in contact. The effective power of the combination is
A) 4.5 D
B) 18 D
C) 45 D
D) 9 D
Explanation
(d) $P_{1} = \frac{100}{20} = 5\ D,\ P_{2} = \frac{100}{25} = 4D$
Effective power $P = P_{1} + P_{2}$
= 5 + 4 = 9 D
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.