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A plano-convex lens has refractive index 1.5 and the radius of curvature of its curved face is 30 cm. The curved face is silvered, and the lens is then used to obtain an image of an object. At what distance from the lens should the object be placed so that the image formed is real and equal in size to the object?

1. 60 cm
2. 30 cm
3. 20 cm
4. 80 cm

Correct answer: 20 cm

Solution

The object must be placed at a distance equal to twice the focal length of the lens to produce a real image that is the same size as the object. Given the radius of curvature and the refractive index, the focal length can be calculated, leading to the conclusion that the object should be placed 20 cm from the lens.

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