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

  • A
    $f_1+f_2$
  • B
    $\frac{f_1 f_2}{f_1+f_2}$
  • C
    $\frac{1}{2}(f_1+f_2)$
  • D
    $\frac{f_1+f_2}{f_1 f_2}$

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