An object 3.0 cm high is placed perpendicular to the principal axis of a concave lens of focal length 7.5 cm. The image is formed at a distance of 5 cm from the lens. Calculate (i) distance at which object is placed and (ii) size and nature of image formed.
Given $h _{ 0 }$> object size = 3 cm,f = -7.5 cm, v = -5 cm, u = ?, I =?
Using lens formula 1/v-1/u = 1/f
We have 1/u = 1/v-1/f = 1/-5-1/-7.5 = -3+2/15 = -1/15
u = -15 cm
The object should be placed 15 cm from the concave lens,
m = v/u = Image size/Object size
Image size = v/u x Object size
I = -5/-15 x 3 = 1 cm
The image is virtual and erect and has a size 1 cm.