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 ($F_C$) = ∞; power of combination ($P_C$) = 0
Given:
Converging lens → focal length $f_1 = +f$
Diverging lens → focal length $f_2 = -f$
When two lenses are placed in contact, the equivalent focal length is:
$\frac{1}{F_C} = \frac{1}{f_1} + \frac{1}{f_2}$
Substitute values:
$\frac{1}{F_C} = \frac{1}{f} + \frac{1}{-f} = 0$
⇒ $F_C = ∞$
Power of combination:
$P_C = \frac{1}{F_C} = 0$
Final Answer:
Focal length $(F_C) = ∞$ ; Power of combination $(P_C) = 0$