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A rod of length 10 cm lies along the principal axis of a concave mirror of focal length 10 cm in such a way that its end closer to the pole is 20 cm away from the mirror. The length of the image is:

1. 10 cm
2. 2.5 cm
3. 15 cm
4. 5 cm

Correct answer: 5 cm

Solution

The image of the rod is formed by considering the magnification at each end. For the end at 20 cm, magnification is -1/2, and for the end at 30 cm, magnification is -1/3. The difference in image positions gives the length of the image as 5 cm.

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