Two thin lenses of focal lengths f1 and f2 are in contact and coaxial. The power of the combination is

Question

Two thin lenses of focal lengths f₁ and f₂ are in contact and coaxial. The power of the combination is

${{{f_1} + {f_2}} \over 2}$

${{{f_1} + {f_2}} \over {{f_1}{f_2}}}$

$\sqrt {{{{f_1}} \over {{f_2}}}} $

$\sqrt {{{{f_2}} \over {{f_1}}}} $

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