41. Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact and coaxial. The power of the combination is ?

41. Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact and coaxial. The power of the combination is ?

(a) \( \sqrt\frac{f_1}{f_2} \)

(b) \( \sqrt\frac{f_2}{f_1} \)

(c) \( \dfrac{f_1 + f_2}{2} \)

(d) \( \dfrac{f_1 + f_2}{f_1 f_2} \)

Answer :  (d) \( \dfrac{f_1 + f_2}{f_1 f_2} \)

\(\dfrac{1}{f} = \dfrac{1}{f_1} + \dfrac{1}{f_2}\)

\(p = \dfrac{f_1 + f_2}{f_1 \cdot f_2}\)