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Power and Thin Lenses in Contact

Two lenses wi...

Question

Two lenses with powers; 2D & -4D are kept together. What is the effective focal length of the combination?

A

40 cm

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B

-50 cm

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C

100 cm

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D

50 cm

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Solution

The correct option is B
-50 cm

Effective Power = $P_{1} + P_{2}$
= 2D + (-4D)
= -2D
$∴ f = \frac{1}{2} m = \frac{- 1}{2} \times 100 = - 50 c m$

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