Question

At what distance should an object be placed from a lens of focal length 25 cm to obtain its image on a screen placed on the other side at a distance of 50 cm from the lens? What will be the magnification produced in this case?

Answer 1

Given that,

Focal length, f = 25 cm

Image distance, v = 50 cm

Form the lens formula,

`1/v-1/u=1/f`            (where u = object distance)

`"or", 1/u=1/v-1/f`

`=1/50-1/25`

Therefore,

`1/u=(-1)/50`

Or, u = −50 cm

Magnification, `m=v/u`

`=50/(-50)`

= -1

The object must be placed 50 cm away from the lens, on the other side to produce a magnification of −1.