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A transparent solid is invisible in a liquid of same refractive index because of no refraction.

1. 1/f = [μg - μm] (1/R1 - 1/R2)
2. μg = μm, then 1/f = (1 - 1) (1/R1 - 1/R2)
3. 1/f = 0
4. f = 1/∞ = 0

Correct answer: 1/f = 0

Solution

When the refractive index of the solid (μg) matches that of the liquid (μm), there is no refraction at the interface, leading to no focusing effect, and the focal length becomes infinite (1/f = 0).

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