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Asked: 2025-09-27T08:43:29+00:00 In:

To get an image of magnification – 1 on a screen using a lens of focal length 20 cm, the object distance must be:

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Poll Results

14.29%(a) less than 20 cm ( 1 voter )
0%(b) 30 cm
85.71%(c) 40 cm ( 6 voters )
0%(d) 80 cm
Based On 7 Votes

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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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    1. ANSWER: [C]

      Explanation: Correct option: (c) 40 cm. 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.

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