The focal length of a convex lens is $20 c m .$ If an object is kept at a distance of $100 c m$ from the lens, find out the distance of the image formed from the lens.

10 c m

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15 c m

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20 c m

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25 c m

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Solution

The correct option is D $25 c m$
Given:
Focal length, $f = 20 c m$
Note that focal length is positive for convex lens.
Object distance, $u = - 100 c m$

Let image distance be $v$ .

Using lens formula:
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$
$\frac{1}{v} - (\frac{1}{- 100}) = \frac{1}{20}$
$v = 25 c m$

Therefore, image distance is $25 c m$ .

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