A teacher sets up the stand carrying a convex lens of focal length 15 cm at 42.7 cm mark on the optical bench. He asks four students A, B, C and D to suggest the position of screen on the optical bench so that a distinct image of a distant tree is obtained almost immediately on it. The positions suggested by the students. As the correct position of the screen was suggested by :

The incident rays coming from the distant tree placed will be parallel to the principal axis and as we know the rays parallel to the principal axis, after refraction by convex lens, will pass through the principal focus.
Therefore, a distinct image will be obtained immediately when distance between screen and lens is equal to focal length:
= 42.7 cm (position of lens on optical bench) + 15 cm (focal length of lens)
= 57.7 (the position of screen on optical bench)