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>> $A convex lens of focal length 40 cm is i$

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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Correct option is C)

Given,

$f_{1} = + 40 c m$

$f_{2} = - 25 c m$

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

The power of combination,

$P = \frac{1}{f _{1}} + \frac{1}{f _{2}}$

$P = \frac{1 0 0}{+ 4 0 c m} + \frac{1 0 0}{- 2 5 c m}$

$P = - 1.5 D$

The correct option is C.

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