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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?
Solution
Focal length, f = +25 cm
Image is virtual and magnified, m = +2
For a lens, magnification is
m = `"v"/"u"`
`therefore + 2 = "v"/"u"`
∴ v = 2u
Lens formula is,
`1/"v" = 1/"u" = 1/"f"`
`therefore 1/"2u" - 1/"u" = 1/25`
`therefore -1/"2u" = 1/25`
∴ 2u = -25 cm
∴ u = -12.5 cm
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