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Question
The focal length of a convex lens is 25 cm. At what distance from the optical centre of the lens an object be placed to obtain a virtual image of twice the size?
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Solution
Focal length, f = +25 cm
Image is virtual and magnified, m = +2
For a lens, magnification is
m = `v/u`
∴ + 2 = `v/u`
∴ v = 2u
Lens formula is,
`1/v = 1/u = 1/f`
∴ `1/(2u) - 1/u = 1/25`
∴ `-1/(2u) = 1/25`
∴ 2u = −25 cm
∴ u = `-25/2`
∴ u = −12.5 cm
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