A converging lens and a diverging lens of equal focal lengths are placed coaxially in contact. The focal length and power of this combination will be
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Focal length (\( F_C \)) = \( \infty \); power of combination (\( P_C \)) = 0
For thin lenses in contact the powers add, with the sign convention that a converging lens has positive focal length and a diverging lens negative.
\( \displaystyle P_C = P_1 + P_2 = \frac{1}{f} + \frac{1}{-f} = 0 \)
\( \displaystyle F_C = \frac{1}{P_C} = \infty \)
The combination behaves like a plane glass plate: it neither converges nor diverges the light.