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To get an image of magnification – 1 on a screen using a lens of focal length 20 cm, the object distance must be:
To obtain an image of magnification M = − 1, the image must be real, inverted and of the same size as the object. This happens when the object is placed at twice the focal length of a convex lens. The given focal length is f = 20 cm. Hence, the object should be placed at 2f = 40 cm. At this position, the image is formed on the opposite side of the lens, also at 40 cm, with the same size as the object but inverted. Correct option: (c) 40 cm.
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