A diverging lens having a focal length of 20 ext cm and a converging lens having a focal length of 30 ext cm are positioned 15 ext cm apart, with their principal axes coinciding. Where must an object be situated on the principal axis in order for its final image to be formed at infinity?

Step-by-step solution

Considering the case where light passes through the diverging lens first, then the converging lens. For the final image to be at infinity, the image formed by the diverging lens must lie at the first principal focus of the converging lens. The focal length of the converging lens is f₂ = +30 cm . The focus of the converging lens is 30 cm in front of it. Since the diverging lens is placed 15 cm in front of the converging lens, the image formed by the diverging lens must be at 30 - 15 = 15 cm in front of the diverging