Calculate the focal length of a convex lens which produces a virtual image at a distance of 50cm of an object placed 20cm in front of it.
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v = -50cm (Virtual image) u = -20cm$\frac{1}{\text{v}}-\frac{1}{\text{u}}=\frac{1}{\text{f}}$
$\frac{1}{-50}-\frac{1}{-20}=\frac{1}{\text{f}}$
$\frac{-2+5}{100}=\frac{1}{\text{f}}$
$\frac{3}{100}=\frac{1}{\text{f}}$
$\text{f}=33.3\text{cm}$
$\frac{1}{-50}-\frac{1}{-20}=\frac{1}{\text{f}}$
$\frac{-2+5}{100}=\frac{1}{\text{f}}$
$\frac{3}{100}=\frac{1}{\text{f}}$
$\text{f}=33.3\text{cm}$
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