A concave lens has focal lengths of 20 cm. At what distance from the lens a 5cm tall object be placed so that it forms an image at 15 cm from the lens ? Also calculate the size of the image formed.

Explanation

`f=-20 cm, h_(0)=5cm, v=-15 cm`
Using `(1)/(f)=(1)/(v)-(1)/(u)` we get
`(1)/(u)=(1)/(v)-(1)/(f)=(1)/(-15)-(1)/((-20))`
`(1)/(u)=(1-20+15)/(300)-(-5)/(300)`
`u=-60 cm`
since `m=(h_(1))/(h_(0))=(v)/(u)`, We get
`h_(1)=(v)/(u)h_(0)=(-15)/(-16)xx 5`
`=(5)/(4)=1.25 cm`
Image.s diminished and virtual.