If the absolute refractive index of a medium is less than 1, it means .
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A.
the medium absorbs all light
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B.
light travels slower in that medium than in vacuum
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C.
light travels faster in that medium than in vacuum
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D.
the medium is a perfect reflector
Correct Answer:
C. light travels faster in that medium than in vacuum
Explanation:
The refractive index ($n$) of a medium is defined by the ratio of the speed of light in a vacuum ($c$) to its phase velocity in that medium ($v$), expressed as $n = c/v$. If the refractive index is less than 1, it mathematically indicates that the phase velocity ($v$) is greater than the speed of light in a vacuum ($c$). While the theory of relativity states that information and matter (group velocity) cannot exceed $c$, the phase velocity—which represents the speed of the wave's crests—can move faster than $c$ in specific environments like plasmas (e.g., the Earth's ionosphere) or for X-rays. Therefore, a refractive index below unity signifies that light's phase velocity in that medium exceeds its speed in a vacuum.
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