An object is placed at $25 c m$ in front of a convex lens. Its image is formed at $75 c m$ from the lens on the same side as the object. Find the power of the lens?

3 D

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2 D

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2.66 D

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3.8 D

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Solution

The correct option is C $2.66 D$
Given:
Object distance, $u = - 25 c m$
Image distance, $v = - 75 c m$

Let the focal length be $f$ and power be $P$ .

Using the lens formula and the formula for power of the lens:
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f} = P$
$P = \frac{1}{- 75 / 100} - \frac{1}{- 25 / 100}$
$P = \frac{8}{3} = 2.66 D$

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