Question

An object is placed 40 cm from a convex lens of focal length 30 cm if a concave lens of focal length 50 cm is introduced between the convex lens and the image formed such that it is 20 cm from the convex lens .Find the change in the position of the image ?

Answer 1

Hey !!

For the convex lens, f₁ = +30 cm and object distance u₁ = - 40 cm.

        1 / f₁ = 1/v₁ - 1/u₁

      => 1 / +30 = 1/v₁ - 1/-40

       => 1/v₁ = 1/30 - 1/40

       => 1/120

=> v₁ = +120 cm, a real image is formed

       On introducing a concave lens, f₂ = -50 cm

and u₂ = 120 - 20 = +100 cm from the concave lens

            1/f₂ = 1/v₂ - 1/u₂

       = 1/-50 = 1/v₂ - 1/+100

       = v₂ = - 100 cm

A virtual image is formed at all the distances of 100 cm from the concave lens.

The change in position between the real image and the virtual image is 100 cm + 100 cm = +200 cm to the left of its original position.

Good luck !!

Answer 2

real image 120 cm from lens