To Find focal length of a convex lens in laboratory, Manoj fixed it on a stand and kept it on a mark of 15.3 cm an optical bench. To get a clear image of a distant tree, he adjusted a screen and finally got clear image when screen was placed at 32.5 cm. Focal length of the lens is :
(a) 32.5 cm (b) 17.2 cm (c) 34.4 cm (d) 47.8 cm

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Solution

When the object distance is equal to infinity, then the image distance is equal to focal length of the convex lens.

Usually, distant image is considered to be object at infinity so that the focal length of the lens is equal to image distance.

The distance between lens and image is equal to difference of their positions on the optical bench, that is, 32.5 - 15.3 = 17.2 cm

Hence, the focal length of the convex lens is equal to 17.2 cm

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