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Power and Thin Lenses in Contact

Two lenses of...

Question

Two lenses of power $+ 12.5 D$ and $- 2.5 D$ are combined to form a compound lens. Its focal length in cm will be

A

10

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B

- 40

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C

- 80

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D

160

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Solution

The correct option is A $10$
Power $1 = + 12.5 D$ , Power $2 = - 2.5 D$ are combined to form a lens.
The combined power is $= + 12.5 D - 2.5 D$
$= + 10 D$
$P = 1 / f$ in metre
$10 = 1 / f m$
$f = 0.1 m$
$f = 0.1 \times 100$
$f = 10 c m$

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