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

Answer 1

Answer: (B)

$5D$

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

$⟹f=20cm=0.2m$

Power of combination $P=\frac{1}{f}=\frac{1}{0.2}=5D$