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Q.
An object is placed at a distance of $10 \,cm$ from a convex lens of power $5D$. Find the position of the image :
Rajasthan PMTRajasthan PMT 2006
Solution:
The focal length of the convex lens
$f=\frac{1}{P} m$
or $f=\frac{1}{5} \times 100 \,cm$
or $f=20\, cm$
From the relation, we get
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
Here, $f=20\, cm , u=-10\, cm$
Therefore, $\frac{1}{v}=\frac{1}{u}+\frac{1}{f}$
$=\frac{1}{-10}+\frac{1}{20} $
$=\frac{-2+1}{20}=-\frac{1}{20}$
Hence, $v=-20 \,cm$
Therefore, the image will be formed at a distance of $20 \,cm$ on the same side of the object.