The focal length of a convex mirror is 40 cm. A point source of light is kept at distance 40 cm from the mirror. Find the distance of image from the mirror.

10 cm

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20 cm

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30 cm

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40 cm

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Solution

The correct option is B 20 cm
Given, focal length, $f = 40 c m$ and object distance $u = - 40 c m$ .
Let the distance of image from the mirror be $v$ .
Using mirror formula,
$\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$
$\Rightarrow \frac{1}{v} = \frac{1}{f} - \frac{1}{u}$
$\Rightarrow \frac{1}{v} = \frac{1}{40} - \frac{1}{- 40}$
$\Rightarrow \frac{1}{v} = \frac{1}{40} + \frac{1}{40} = \frac{1}{20}$
or $v = 20 c m$
Therefore, the image is formed at a distance of 20 cm from mirror.

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The focal length of a convex mirror is 40 cm. A point source of light is kept at distance 40 cm from the mirror. Find the distance of image from the mirror.

The correct option is B 20 cmGiven, focal length, f=40 cm and object distance u=−40 cm. Let the distance of image from the mirror be v. Using mirror formula, 1f=1u+1v ⇒1v=1f−1u ⇒1v=140−1−40 ⇒1v=140+140=120 or v=20 cm Therefore, the image is formed at a distance of 20 cm from mirror.