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Two thin lenses of focal lengths f₁ and f₂ are in contact and coaxial. The power of the combination is:

1. f₁ / f₂
2. f₂ / f₁
3. f₁ + f₂ / 2
4. f₁f₂

Correct answer: f₁f₂

Solution

The power of a combination of two thin lenses in contact is given by the sum of their individual powers, P = P₁ + P₂. Since power is the reciprocal of focal length (P = 1/f), the combined power is P = 1/f₁ + 1/f₂, which simplifies to f₁f₂ / (f₁ + f₂).

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