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Question:

A student has obtained an image of a distant object on a screen to determine the focal length F1 of the given lens. His teacher after checking the image, gave him another lens of focal length F2 and asked to focus the same object on the same screen. The student found that to obtain a sharp image he has to move the lens away from the screen. From this finding we may conclude that both the lenses given to the student were:

Convex and F1>F2

Concave and F1<F2

Convex and F1<F2

Concave and F1>F2

Solution:

As the obtained image is a real image, so it is a convex lens. Image is formed at the focus of the mirror as the object is far away. As object distance u is the same in both cases and image distance for the lens having focal length F2 is increasing as the student is moving the lens away from the screen. Therefore, the focal length of the second lens F2 will be more as compared to the focal length of F1.