A diverging lens of focal length 20 cm and a converging lens of focal length 30 cm are placed 15 cm apart with their principal axes coinciding. Where should an object be placed on the principal axis so that its image is formed at infinity?

focal length of convex lens, = 30cm

focal length of Concave lens, = 15 cm

distance between both the lens, d = 15 cm

the equivalent focal length of both the lens is given by

The equivalent lens is a converging lens as focal length is positive.

Distance from diverging lens so that emergent beam is parallel (image at infinity),

It should be placed 60 cm left to diverging lens.

Object should be placed (120 – 60) = 60 cm from diverging lens.

Similarly, distance from converging lens

So, it should be placed 90 cm right to converging lens.

Object should be placed (120 + 90) = 210 cm right to converging lens.