Question

You are given three lenses L1, L2 and L3 each of focal length 20cm. An object is kept at 40cm 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

Given f1 = f2 = f3 = 20cm
For lens L1
u1 = – 40cm

Thus lens L2 should produce image at infinity.

Hence, for L2, its objective should be at focus. The image formed by lens L1 is at 40cm on the right side of lens L1 which at 20cm left of lens L2 i.e., focus of lens L2. Hence, the distance between L1 and L2 = 40 + 20 = 60cm. As the image formed by lens L2 at infinity, then, the distance between lens L2 and L3 does not matter.

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