Question

Two thin lenses of focal length $f_1=10\text{cm}$ and $f_2=20\text{cm}$ are kept as shown. The resultant power of combination will be

• A. $-10\text{D}$
• B. $0$
• C. $10\text{D}$
• D. $5\text{D}$

Answer 1

The correct option is D $5\text{D}$

Given :
$f_1=10\text{cm}$,
$f_2=20\text{cm}$

Focal length of plane mirror $f_3=\infty$

The resultant power of the combination is

$\frac{1}{f}=\frac{1}{f_1}-\frac{1}{f_2}+\frac{1}{f_3}$

$\frac{1}{f}=\frac{1}{10}-\frac{1}{20}+\frac{1}{\infty }$

So, $\frac{1}{f}=\frac{1}{20}$

$\Rightarrow f=20\text{cm}=0.2\text{m}$

Power of combination

$P=\frac{1}{f}=\frac{1}{0.2}=5\text{D}$

Hence option $\text{(B)}$ is the correct answer.