Q. Two thin lenses of focal length 20 cm and 25 cm are placed in contact .The effective power of the combination is
A) 9 D
B) 2 D
C) 3 D
D) 7 D
Explanation
(a) $P = P_{1} + P_{2}$
$= \frac{1}{f_{1}} + \frac{1}{f_{2}} = \frac{100}{20} + \frac{100}{25}$
$= 5 + 4 = 9D$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.