Question

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

• A. – 1.5 D
• B. – 6.5 D
• C. + 6.5 D
• D. + 6.67 D

Answer 1

The correct option is A – 1.5 D

By using $\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}\Rightarrow \frac{1}{F}=\frac{1}{+40}+\frac{1}{-25}$
$\Rightarrow F=-\frac{200}{3}cm$, hence $P=\frac{100}{f\left(cm\right)}=\frac{100}{\frac{-200}{3}}=-1.5D$