The image obtained iwth a convex lewns is ereect and its length is four times the length of the object.If the focal length of the lens is 20 cm,calculate the object and image distance.

Explanation

`:.` Image formed by lens is erect and its length is 3 times the length of object
`:.` Given lens is a convex lens
`impliesf=+15` cm
m=3
`m=v/u`
`3=v/u`
`impliesv=3u`
We know `1/v-1/u=1/f`
`1/(3u)-1/u=1/15`
`(-13)/(3u)=1/15`
`(-2)/(3u)=1/15`
`u=-30/3=-10cm`
`impliesv=3u=3(-10)=-30cm`