Problems on Thin Lens

Physics

example

Image formation from lenses where object is being moved

Example: A thin converging lens of focal length forms the image of an object on a screen placed at a distance of from the lens. The screen is moved closer to the lens by a distance of . Find the distance through which the object has to be shifted so that its image on the screen is sharp again.

Solution:
Focal length of the converging lens:
Case 1: Image distance: .
Let object distance be
Using , we get:

or,
This means that the object is at a distance of from the lens initially.
Case 2: The screen is shifted by towards the lens.
New image distance:
Let new object distance be
Using: , we get:

or,
 So, the object has to be shifted by a distance:  away from the lens.

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