Q. Two thin lenses have a combined power of + 9D. When they are separated by a distance of 20 cm, their equivalent power becomes $+ \frac{27}{5}\ D.$ Their individual powers (in dioptre) are
A) 4, 5
B) 3, 6
C) 2, 7
D) 1, 8
Explanation
(b) $P_{1} + P_{2} = 9$
$P = \ P_{1} + P_{2} - dP_{1}P_{2}$
$\frac{27}{9} = 9 - \frac{20}{100} \times P_{1}P_{2}\ $
The above equation is correct for $P_{1} = 3\ and\ P_{2} = 6$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.