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

Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact and coaxial. The power of the combinations is
  1. $\frac{f_1+f_2}{f_1 f_2}$
  2. $\sqrt{\frac{f_1}{f_2}}$
  3. $\sqrt{\frac{f_2}{f_1}}$
  4. $\frac{f_1+f_2}{2}$

Solution

$\begin{aligned} & \frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2} \\ \Rightarrow \quad P & =\frac{f_1+f_2}{f_1 f_2} \end{aligned}$

Asked in: NEET 2008 (Mains)

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