Why is it important that a light-year is defined by light traveling in a vacuum?
Answer
Light slows down when moving through substances like water or atmosphere
Light interacts with particles when passing through materials such as glass, water, or the atmosphere, which causes it to slow down. To maintain a constant, unwavering definition, a light-year must be based on light traveling through a vacuum, where it encounters no interference and maintains its maximum possible speed.
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How Many Years are in a Light Year? | The Speed of Light - YouTube
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