Question

A student 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 A screen. From this finding we may conclude that both the lenses given to the student were:; A. Concaveand F1 F2; D. Concave and F1 > F2(Delhi - 2014)

Answer 1

Answer:

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Answer 2

By this experiment, we can conclude that both lenses were

Convex and F1>F2

1. It is not mentioned what the nature of the image is so we assume it to be convex. Therefore,

• The lens is convex.

    2.Now, in convex lens, the lens formula is

• $f=\frac{vu}{u-v}$
• object distance=-u
• image distance =v
• Therefore, focal length=$\frac{vu}{v+u}$

Therefore, the value of F is less for the second lens.

• Hence, F1>F2.