How could you find the focal length of a convex lens rapidly but approximately?

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Solution

Put a paper and put the lens in front of it and capture the image of a distant object by moving the lens to and fro. When a sharp and inverted image is formed, measure the distance between the lens and the paper. That is a rapid and approximate method of measuring the focal length of the lens. Since u the distance from the object to the lens is large, 1/u can be neglected and that equation can be written as 1/v = 1/f and thus v = f

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