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The same size images are formed by a convex lens when the object is placed at 20cm or at 10cm from the lens. The focal length of convex lens is ____ cm.

1. 5
2. 10
3. 15
4. 20

Correct answer: 15

Solution

The focal length of a convex lens can be determined using the lens formula. When the object distances are at 20 cm and 10 cm, the image distances will be equal due to the same size images formed, allowing us to derive that the focal length is 15 cm.

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