Question

An object 3cm high is placed perpendicular to the principal axis of a concave lens of focal length 15cm
The image is formed at a distance of 10cm from the lens. Calculate i) Object distance ii) Size and
nature of the image formed​

Answer 1

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.