Question

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

Answer 1

Here, `f = -20 cm`
`u =?, h_(1)=5cm` `v = -15cm`, `h_(2)=?`
As `(1)/(v) - 1/u = (1)/(f) :. 1/u = (1)/(v) - (1)/(f) = 1/-15 + 1/20 =(-1)/(60)`
`u = -60cm`
From `m=(h_(2))/(h_(1))=v/u, (h_(2))/(5) = (-15)/(-16)=1/4, h_(2) = 5/4cm =1.25cm`
This is the height of virtual image.