Question 38: Two thin lenses are of same focal lengths (f), but one is convex and the other one is concave. When they are placed in contact with each other, the equivalent focal length of the combination will be :
(1) f / 2
(2) Infinite
(3) Zero
(4) f / 4
Answer: Option (2)
Explanation:
For thin lenses placed in contact, the equivalent focal length is given by the relation
\frac{1}{F}=\frac{1}{f_{1}}+\frac{1}{f_{2} }.
The focal length of the convex lens is f_{1}=+f and
the focal length of the concave lens is f_{2}=-f.
Substituting these values, \frac{1}{F}=\frac{1}{f}+\frac{1}{-f}.
This gives \frac{1}{F}=0.
Hence, the equivalent focal length F is infinite.
This means the combination neither converges nor diverges light.
Therefore, the correct answer is Option (2).