Question

A student obtained a clear image of window grills on the screen. But the teacher told him to get the image of a tree far away, instead of the window. To get a clear image the lens must be:
A) Moved towards the screen
B) Moved away from the screen
C) Moved very close to the screen
D) None of these.

Answer 1

Hint: from the above question it is clear that the distance of the tree from the lens is greater than the distance of the window grills. Now a lens is kept between the window grills and a screen and the student getting a clear image of the grills. The lens should be converging as the tree is far away from the grills. In the converging lens, the nearer the object the more distance the image from the lens. Therefore for the faraway tree, the image will be near the lens.

Complete solution:
Step 1: First let us understand how a converging lens forms the image and how the position of the object affects the position of the image. Consider the following diagram.
In the ray diagram window grills, AB is closer to the lens in comparison to the tree CD. Since the student is getting a clear image of the window grill therefore the screen kept by the student will be at the same distance as the distance of the image of the window grills A’B’.
Step 2: From the above diagram it is also clear that when the student was getting a clear image of the window grills then the image of the tree which is far away from the grills was being formed between the screen and the lens. The screen is fixed. If we can only move the lens then it is pretty clear that the lens should be moved towards the screen to get a clear image of the tree. Then only the image C’D’ can be found clearly on the screen.

Hence the correct option is Option A.

Note: The lens cannot be moved away from the screen if we want a clear image of the tree. Because the distance of the image from the lens of an object depends on the distance of the object from the lens. In the case of the converging lens the far the object is kept nearer the image is formed. If we took the lens away from the screen then the image of the tree will go away from the screen.