A concave lens of 20cm focal length forms an image 15cm from the lens. Compute the object distance.
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u = -20cm v = -15cm (Virtual image) Lens formula, $\frac{1}{\text{f}}=\frac{1}{\text{v}}-\frac{1}{\text{u}}$$\frac{1}{-15}-\frac{1}{\text{u}}=\frac{1}{-20}$
$\frac{1}{\text{u}}=-\frac{1}{15}+\frac{1}{20}$
$\frac{1}{\text{u}}=\frac{-1}{60}$
$\text{u}=-60\text{cm}$
Object distance is 60cm towards the left of the lens.
$\frac{1}{\text{u}}=-\frac{1}{15}+\frac{1}{20}$
$\frac{1}{\text{u}}=\frac{-1}{60}$
$\text{u}=-60\text{cm}$
Object distance is 60cm towards the left of the lens.
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