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 the 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  




In a convex lens focal length F = 15 cm ,

  • Here, 42.7 cm is where the screen is, NOT where the object is

  • Object is a distant tree, so, object is at infinity.

  • Since object is at infinity, image is formed at Focus on the right side of lens.
    Convex lens of focal length 15 cm at 42.7 cm mark on the optical bench.jpg

  • Thus, screen will be moved 42.7 cm to the right , to reach Optical center.

  • And then, 15 cm more to the right to reach Focus on right side.

  • Thus, new position will be 42.7 cm + 15 cm = 57.7 cm

So, the correct answer is (c) 

Ask a doubt
Maninder Singh's photo - Co-founder, Teachoo

Made by

Maninder Singh

CA Maninder Singh is a Chartered Accountant for the past 14 years and a teacher from the past 18 years. He teaches Science, Economics, Accounting and English at Teachoo