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Power and Thin Lenses in Contact

Question

A convex lens of focal length 40 cm is in contact with a concave lens of focal length 25 cm. The power of he combination, is :

A

+ 6.67 D

B

- 6.5 D

C

- 1.5 D

D

+ 6.5 D

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Solution

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Given,

$f_{1} = + 40 c m$

$f_{2} = - 25 c m$

Power, $P = \frac{1}{f}$

The power of combination,

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

$P = \frac{100}{+ 40 c m} + \frac{100}{- 25 c m}$

$P = - 1.5 D$

The correct option is C.

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