Question

You are given three lenses L₁, L₂  and L₃  each of focal length 10cm. An object is kept at 15cm in front of L₁, as shown. The final real image is formed at the focus ‘I’ of L₃. Find the separations between L₁, L₂  and L₃.

Answer 1

Since, final image is formed by lens L₃ at focus. Hence, for L₃, the object must be at infinity. Thus lens L₂ should produce image at infinity. So for L₂, its object should be at focus. 

The image formed by lens L, is at 15cm on the right side of lens L₁ which lies at 10cm left of lens L₂, i.e., focus of lens L₂. 

Hence, distance between L₁ and L₂ = 30cm + 10cm = 40cm 

As the image formed by Lens L₂ lies at infinity. Then, the distance between lens L₂ and L₃ does not matter.

 Hence, the distance between L₂ and L₃ can have any value.