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
Answer
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.

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)