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Question
A lens forms the image of an object placed at a distance 15 cm from it, at a distance 60 cm in front of it. Find the focal length.
Solution
Object distance, u = -15 cm
Image distance, v = -60 cm
Lens formula is,
`1/v - 1/u = 1/f`
∴ `1/-60 - 1/-15 = 1/f`
∴ `1/f = 1/15 - 1/60`
∴ `1/f = 3/60`
∴ `1/f = 1/20`
∴ f = 20 cm
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