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A convex lens and a concave lens, each having same focal length of 25 cm, are put in contact to form a combination of lenses. The power in diopters of the combination is

1. 50
2. infinite
3. zero
4. 25

Correct answer: zero

Solution

The power of a lens combination is the algebraic sum of the individual powers. Since the convex lens has a power of +4D (1/0.25) and the concave lens has a power of -4D (1/(-0.25)), their sum is 0D, making the combination's power zero.

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