A lens of focal length 20cm is used to produce a ten times magnified image of a film slide on a screen. How far must the slide be placed from the lens ?
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f = 20cm m = -10 (Image is real) u = ?$\text{m}=\frac{\text{v}}{\text{u}}=-10$
$\text{v}=-10\text{u}$
Lens formula, $\frac{1}{\text{v}}-\frac{1}{\text{u}}=\frac{1}{\text{f}}$$\frac{1}{-10\text{u}}-\frac{1}{\text{u}}=\frac{1}{20}$
$\frac{-1-10}{10\text{u}}=\frac{1}{20}$
$\frac{-11}{10\text{u}}=\frac{1}{20}$
$\text{u}=-22\text{cm}$
$\text{v}=-10\times-22=220\text{cm}$
$\text{v}=-10\text{u}$
Lens formula, $\frac{1}{\text{v}}-\frac{1}{\text{u}}=\frac{1}{\text{f}}$$\frac{1}{-10\text{u}}-\frac{1}{\text{u}}=\frac{1}{20}$
$\frac{-1-10}{10\text{u}}=\frac{1}{20}$
$\frac{-11}{10\text{u}}=\frac{1}{20}$
$\text{u}=-22\text{cm}$
$\text{v}=-10\times-22=220\text{cm}$
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