Question

Two thin lenses of focal lengths 20 cm and -20 cm are placed in contact with each other. The combination has a focal length equal to

• A. Infinite
• B. $50cm$
• C. $60cm$
• D. $10cm$

Answer 1

Answer: (A)

Infinite

$\begin{array}{cc}{f}_1=20cm& ,f_2=-20cm\\ \frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}& [f=\text{equivalent}focallength\end{array}]$

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

$\Rightarrow \frac{1}{f}=0=>f=\infty$

$\therefore$ Required answer $=\infty$

$=$ infinite