Question

Two thin lenses have a combined power of $+9 D$. When they are separated by a distance of $20\, cm$, their equivalent power becomes $+\frac{27}{5} D$. Their individual powers (in diopters) are

• A. 1,8
• B. 2,7
• C. 3,6
• D. 4,5

Answer 1

Answer: (C)

3,6

Answer 2

Given,
$P_{1}+P_{2}=9$
$P=P_{1}+P_{2}-d P_{1} P_{2}$
$\frac{27}{5}=9-\frac{20}{100} \times P_{1} P_{2}$
Hence, the above equation is correct for
$P_{1}=3$ and $P_{3}=6$