Question

The image obtained with a convex lens is erect and its length is `4` times the length of the object. If focal length of the lens is `20cm`, calculate the object and image distances.

Answer 1

As image formed is erect and hence virtual, the magnification produced by the lens is positive, i.e., `m = + 4`
`f= + 20 cm, u = ?`
`4 = (20)/(u + 20), 4u + 80 = 20`
`u = (20 - 80)/(4) = - 15 cm`
Again, from `m = (f - v)/(f)`
`4 = (20 -v)/(20)`
`80 = 20 - v`
`v = 20 - 80 = - 60 cm`