Question

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 were as

A. 12.7 cm

B. 29.7 cm

C. 57.7 cm

D. 72.7 cm

The correct position of the screen was suggested by

(a) A

(b) B

(c) C

(d) D

Answer 1

Here, the object is considered at infinity. Thus, the image will be formed 15 cm away from the lens at its focus. Thus, the screen should be placed at 42.7 + 15 = 57.7 cm mark on the optical bench. 

The correct position of the screen was suggested by C.

Hence, the correct answer is option (c).