Question

a convex lens produces a double sized real image when an object is placed at a distance of 18 cm from it.where should the object be placed to produce a triple size real image?

Answer 1

in Ist case m=-2
       u=-18
    m=v/u
   v=mv
    = -2*-18=36

1/f=1/v-1/u
    = 1/36+1/18
   = 1/12  ..
in case (2) m=-3
let u=-x
v= -3*-x=3x

1/f=1/v-1/u
1/12=1/3x+1/x
 1/12=4/3x
x=16
therefore object is plced at -x==-16 cm