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?

Answer 1

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