Light Speed in Vacuum: What's the Significance?

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The phrase "speed of light in a vacuum" emphasizes that light travels at its maximum speed only in a vacuum, where it is unimpeded. In other mediums, light interacts with atoms, causing it to travel slower due to absorption and re-emission processes. The degree of this slowing varies based on the medium's properties and the frequency of interactions. Understanding this distinction is crucial for grasping the behavior of light in different environments. The significance lies in recognizing that the vacuum represents the ultimate speed limit for light.
Nickriener
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OK, I've been wondering, what's the significance of saying "speed of light in a vaccum" if lightspeed is a constant. Is the "vacuum" part even necessary?
 
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Except as a clarification that light only travels in a vacuum...light appears to travel slower through a medium but that is only because of occasional interactions between light and atoms.
 
such as your frame of reference?
 
Light travels unimpeded in a pure vacuum - where it achieves its maximum velocity. Photons traveling through a medium are absorbed then reemitted by atoms, which slows their journey. The amount of slowing depends on how long the photons are 'held' before reemitted, and the number of captures suffered before reaching an observer.
 
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