Calculate the distance at which an object should be placed in front of a convex lens of focal length 100 cm to obtain an erect image of double its size.

Explanation

f=100 cm
`m=v/u=2`
`v=2u`
`1/f=1/v+1/u`
`1/100=1/(2u)-1/u`
`1/100=(1-2)/24`
`=-1/24`
`:.2u=-100`
`:.u=-50cm`