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  4. An object of height 1.5 cm is placed on the axis of a convex lens of focal length 25 cm. A real image is formed at a distance of 75 cm from the lens. The size of the image will be

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Q. An object of height $1.5 \,cm$ is placed on the axis of a convex lens of focal length $25 \,cm$. A real image is formed at a distance of $75 \,cm$ from the lens. The size of the image will be

Chhattisgarh PMTChhattisgarh PMT 2005

Solution:

Magnification $m=\frac{I}{O}=\frac{-v}{u}$
Given: $O=1.5\, cm$
By, lens formula
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u} $
$\frac{1}{25}=\frac{1}{75}-\frac{1}{u} $
or $u=-\frac{75}{2}\, cm $
$\therefore \frac{I}{1.5}=-\frac{75}{(-75 / 2)} $
or $ I=3\, cm$