Question

A teacher sets up a stand carrying a convex lens of focal length 15cm at 42.7cm 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 distinct tree is obtained almost immediately on it. The positions suggested by the students are 12.7cm, 29.7cm, 57.7cm and 72.7. The correct position of the screen is?

Answer 1

As we know,
A ray comes from infinty , after refraction through a convex lens, meet
at the second principal focus. So, screen should be placed at the second
principal focus of a convex lens.e.g., screen should be placed at 15cm from the pole of convex lens.
Now, the position of the screen on the optical bench = distance between screen from the pole of convex lens + distance of optical bench from the pole of convex lens { as you can see in attachment}
= 15 cm + 42.7cm
= 57.7cm

Hence, option (C) is correct

Answer 2

Option C : 57.7cm is correct option .

Explanation:


Given convex lens of Focal length =f=15cm

It is placed at 42.7cm mark on optical bench[position of lens]

When we want the distant object let us say tree to be focused approximately , we get image at focus of convex lens .
Because rays from distant object are parallel rays and they get converged at focus.

Hence position of screen=Distance marked on bench[lens placed] + focal length
=42.7 +15
=57.7cm