Question

What will be the resultant focal length of the combination when a thin convex lens is kept in contact with a thin concave lens ?

• A. $\infty$
• B. $0$
• C. $\frac{f}{2}$
• D. $f$

Answer 1

The correct option is A $\infty$

Let the focal length of the convex lens be $f$.
Then the focal length of the convex lens is $-f$.
so using $\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}$
i.e., $\frac{1}{f}=\frac{1}{f}+\frac{1}{-f}$
Therefore, $\frac{1}{f}=0$
Thus , $f=\infty$