Question

To find the image-distance for varying object-distance in case of a convex lens, a student obtains on a screen a sharp image of a bright object placed very far from the lens. after that he gradually moves the object towards the lens and each time focuses its image on the screen. in which direction from the lens, does he move the screen to focus the object?

a. away from the lens
b. towards the lens
c. depends on the focal length of the lens
d. depends on the size of the object.

Answer 1

Correct option: a. away from the lens

The screen should be moved away from the convex lens as the object approaches, to keep the image focused, until the object is closer than the lens' focal length, at which point a virtual image is formed and the screen movement is no longer relevant.

Step-by-step explanation:

When using a convex lens to create an image on a screen for varying distances of an object, a student will move the screen in different directions to maintain focus.

Initially, with the object placed very far from the lens, the sharp image obtained is close to the focal plane, and as the object moves closer to the lens, the screen must be moved away from the lens to keep the image focused.

This is because the image distance increases as the object distance (do) decreases.

However, when the object comes closer than the lens' focal length (f), a different scenario occurs which involves a case 2 image.

A case 2 image cannot be projected on a screen; instead, it is virtual, upright, larger than the object, and seen on the same side as the object.

Therefore, for objects placed closer than the focal length, the answer to the student's question would be that the screen does not need to be moved at all, as a focused image cannot be projected on the screen in this scenario.