Question

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

• A. -10 D
• B. 5 D
• C. -5 D
• D. 10 D

Answer 1

Answer: (B)

Given : $f_1=10cm,$
$f_2=20cm$
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 $\frac{1}{f}=\frac{1}{20}$
$\Rightarrow f=20cm=0.2m$
Power of combination
$P=\frac{1}{f}=\frac{1}{0.2}=5D$