If the absolute refractive index of a medium is less than 1, it means ________.
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A.
light travels slower in that medium than in vacuum
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B.
the medium absorbs all light
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C.
the medium is a perfect reflector
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D.
light travels faster in that medium than in vacuum
Correct Answer:
D. light travels faster in that medium than in vacuum
Explanation:
The absolute refractive index ($n$) of a medium is defined by the ratio $n = c/v$, where $c$ is the speed of light in a vacuum and $v$ is the phase velocity of light within that medium. If the refractive index is less than 1, it mathematically indicates that the phase velocity of light in that medium is greater than its speed in a vacuum ($v > c$). This phenomenon, often called anomalous refraction, occurs in specific conditions such as in plasma (like the ionosphere) or with X-rays. While the phase velocity can exceed $c$, the group velocity—which represents the actual speed of information and energy transfer—remains below the speed of light, ensuring no violation of the laws of relativity. Therefore, a refractive index below 1 signifies that light travels faster in that medium than in a vacuum.
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