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At what distance should an object be placed from a convex lens of focal langth `15 cm` to obtain an image three times the size of the object ?

Answer» Here, `u = ?, f = 15cm, m = +-3 = (v)/(u)`
`:. v = +-3 3u`
From `(1)/(v) - (1)/(u) = (1)/(f)`
`(1)/(+- 3u) - (1)/(u) = (1)/(15)`
`:. u = 20 cm`, when `m` is negative,
`u = 10cm`, when `m` is positive,
i.e., for virtual image, `u = 10 cm` and for reqal image, `u = 20cm`


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