An object is placed at a certain distance from a convex lens of focal length 20 cm. Find the distance of the object if the image obtained is. magnified 4 times.

Explanation

f = - 20 cm
v = - 4 u
According to lens formula
`(1)/(f) = (1)/(v) + (1)/(u)`
`(1)/((-20))=(1)/((-4u))+(1)/(u)`
`(1)/((-20))=(1)/(u)[-(1)/(4)+1]`
`=(1)/(u)[(3)/(4)]`
`u=(3xx20)/(4)=-15cm`