A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens?

10 cm

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15 cm

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20 cm

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30 cm

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Solution

The correct option is D
30 cm

Here,

Focal length of concave lens, $f = - 15 c m$

Image distance, $v = - 10 c m$

According to lens formula,

$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$

$∴ \frac{1}{u} = \frac{1}{v} - \frac{1}{f}$ $\Rightarrow \frac{1}{u} = \frac{- 1}{10} - \frac{1}{(- 15)} = \frac{- 1}{10} + \frac{1}{15} = \frac{- 5}{150}$

Hence, $u = - 30 c m$

The negative value of u indicates that the object is placed 30 cm in front of the lens.

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on the same side of the lens, then find the focal length of the lens used.

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