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A concave lens has focal length 15cm. If we want to get a sharp image on a screen placed at 10cm from the lens, then at what distance the object should be placed from the lens ?
Given:
Image distance, $v=-10 \mathrm{cm}$
Focal length, $f=-15 \mathrm{cm} \quad$ [f is -ve for a concave lens]
To find: We have to find the object distance, $u$
Solution:
Using lens formula,
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
we have,
$\frac{1}{u} =\frac{1}{v}-\frac{1}{f} $
$=\frac{1}{-10}-\frac{1}{-15}$
$=\frac{-3+2}{30}$
$=-\frac{1}{30}$
So, $u =-30 \mathrm{cm}$
Thus the object should be placed at a distance of $30 \mathrm{cm}$ from the lens on the left side.
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