Two thin lenses of focal lengths $f_{1}$ and $f_{2}$ are in contact and coaxial. The power of the combinations is

\frac{f_{1} + f_{2}}{f_{1} f_{2}}

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\sqrt{\frac{f_{1}}{f_{1}}}

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\sqrt{\frac{f_{2}}{f_{1}}}

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\frac{f_{1} + f_{2}}{2}

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Solution

The correct option is A $\frac{f_{1} + f_{2}}{f_{1} f_{2}}$
If two thin lenses of focal lengths $f_{1}$ and $f_{2}$ are placed in contact co-axially then equivalent focal length of combination is,

$\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} - \frac{0}{f_{1} f_{2}} = \frac{1}{f_{1}} + \frac{1}{f_{2}}$

Power of the combination is,

$P = \frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} = \frac{f_{1} + f_{2}}{f_{1} f_{2}}$

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Similar questions

f_{1}

f_{2}

If two thin lenses of focal length f₁ and f₂ are held in contact then the focal length of the combination will be

f_{1}

f_{2}

\frac{1}{f} = \frac{1}{f_{1}} + \frac{1}{f_{2}}

∣ ∣ ∣_{1} \times_{2} ∣ ∣ ∣ =_{1} ._{2}

∣ ∣ ∣_{1} +_{2} ∣ ∣ ∣

f_{1}

f_{2}

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