A small object is so placed in front of a convex lens of 5cm focal length that a virtual image is formed at a distance of 25cm. Find the magnification
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f = 5cm v = -25cm (Virtual image) Lens formula, $\frac{1}{\text{v}}-\frac{1}{\text{u}}=\frac{1}{\text{f}}$$\frac{1}{-25}-\frac{1}{\text{u}}=\frac{1}{5}$
$\frac{1}{\text{u}}=-\frac{1}{25}-\frac{1}{5}=-\frac{6}{25}$
$\text{u}=-\frac{25}{6}\text{cm}$
Magnification, $\text{m}=\frac{\text{v}}{\text{u}}=\frac{-25}{-\frac{25}{6}}=+6$
$\frac{1}{\text{u}}=-\frac{1}{25}-\frac{1}{5}=-\frac{6}{25}$
$\text{u}=-\frac{25}{6}\text{cm}$
Magnification, $\text{m}=\frac{\text{v}}{\text{u}}=\frac{-25}{-\frac{25}{6}}=+6$
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