Question

A convex lens of focal length $F$ is placed in contact with a concave lens of focal length $F$. The equivalent focal length of the combination is:

• A. Infinity
• B. $F/2$
• C. $2F$
• D. Zero

Answer 1

The correct option is A Infinity

Given that:

$f_1=F$ & $f_2=-F$

Let the equivalent focal length be $f$.

$\Rightarrow \frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}$

$=\frac{1}{F}+\frac{1}{-F}$

$=0$

Therefore, $f=\frac{1}{0}=\infty$