Question

A convex lens of focal length $40cm$ is in contact with a concave lens of focal length $25cm$. The power of the combination is

• A. $-1.5dioptres$
• B. $-6.5dioptres$
• C. $+6.5dioptres$
• D. $+6.67dioptres$

Answer 1

Answer: (A)

$-1.5dioptres$

$\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}=\frac{1}{0.4}+\frac{1}{-0.25}=\frac{1}{0.4}-\frac{1}{0.25}$
$=\frac{0.25-0.4}{0.4\times 0.25}=\frac{-0.15}{0.4\times 0.25}$
$\frac{1}{f}=\frac{0.15}{0.4\times 0.25}=-1.5$
$\Rightarrow P=\frac{1}{f}=-1.5dioptre$.