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)
