If an object is placed at a distance of $30 c m$ and the image is formed at a distance of $10 c m$ on the same side of the lens, then find the focal length of the lens used.

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 B $15 c m$
Given:
Image distance, $v = - 10 c m$
Object distance, $u = - 30 c m$
[Using the sign conventions, as both object and image are on the left side of the lens] Let focal length be $f$ .

Using the lens formula:
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$
$\frac{1}{- 10} - \frac{1}{- 30} = \frac{1}{f}$
$f = - 15 c m$
Here negative sign shows that the lens is concave.

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If an object is placed at a distance of 30 cm and the image is formed at a distance of 10 cm on the same side of the lens, then find the focal length of the lens used.

If an object is placed at a distance of $30 c m$ and the image is formed at a distance of $10 c m$ on the same side of the lens, then find the focal length of the lens used.