Three students measured the focal length of a convex lens using parallel rays from a distant object. All of them measured the distance between the lens and the inverted image on the screen.

Student A saw a sharp image on the screen and labelled the distance as f1.

Student B saw a slightly larger blurred image on the screen and labelled the distance as f2.

Student C saw a slightly smaller blurred image on the screen and labelled the distance as f3.

The relation between the three measurements would most likely be:

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f 1 = f 2 = f 3

f 1 < f 2 and f 3

f 3 < f 1 < f 2

f 1 < f 2 and f 1 = f 3

answer is C.

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Detailed Solution

In the above case, the relation between the three measurements would most likely be

f 3 < f 1 < f 2

The focal length is the distance between the pole of the mirror and focus. The convex lens converges the parallel light rays coming from a distant object on the focus. So as a sharp image,

f 1

is the focus point. As student B saw a slightly larger blurred image, so it is formed beyond focus and as student C saw a slightly smaller blurred image, it is formed before focus.

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