Determine how far an object must be placed in front of a converging lens of focal length 10 cm in order to produce an erect (upright) image of linear magnification 4.

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Solution

Here, $f = 10 c m$

$m = 4$

$\Rightarrow \frac{v}{u} = 4$

$\Rightarrow v = 4 u$

Appling lens formula, $\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$

$\Rightarrow \frac{1}{4 u} - \frac{1}{u} = \frac{1}{10}$

$\Rightarrow \frac{- 3}{4 u} = \frac{1}{10}$

$\Rightarrow u = \frac{- 30}{4} = - 7.5 c m$

So, the object should be placed at a distance 7.5 cm from the lens.

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Determine how far an object must be placed in front of a converging lens of focal length 10 cm in order to produce an erect (upright) image of linear magnification 4.

Here, f=10 cm m=4 ⇒vu=4 ⇒v=4u Appling lens formula, 1v−1u=1f ⇒14u−1u=110 ⇒−34u=110 ⇒u=−304=−7.5 cm So, the object should be placed at a distance 7.5 cm from the lens.