# Lightspeed?

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Light has a constant speed in vacuum of space, and a different constant speed in water, etc.
So a light coming from a car standing still is same speed as light from a car that is moving,
because also in the atmosphere lightspeed is a constant .
But, if an observer is standing still to a lightsource or in motion to a lightsource,
speed of light is not the same, while a constant in the atmosphere?

Or not?

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Light in any medium travels at c/n; it's more apparent in something like water, which has a relatively large index (around 1.33) as compared to air (1.000273 at STP)

Fizeau measured the effects of relative motion between source and the medium back when people thought there was an aether (in 1851). But the results were smaller than what he expected, and support relativity and its velocity addition formula

So there is a small effect from the source and medium having relative motion, that changes the speed of light propagation

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The speed would change in different reference frames, exactly as the speed of an object would. To see this, imagine light through some medium, and an object traveling along with it at the same speed, so that they both pass through the same set of events. In another frame, if their speeds were no longer equal, they wouldn't pass through the same events, which would be a paradox. As swansont mentioned, because of the velocity addition (composition) formula, changing the observer's speed by non-relativistic v will change the speed of light in the medium by a LOT less than v.

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13 hours ago, Hello2 said:

Light has a constant speed in vacuum of space, and a different constant speed in water, etc.
So a light coming from a car standing still is same speed as light from a car that is moving,
because also in the atmosphere lightspeed is a constant .
But, if an observer is standing still to a lightsource or in motion to a lightsource,
speed of light is not the same, while a constant in the atmosphere?

Or not?

The speed of light in a vacuum is invariant as it is c.  This not only means that the speed you measure for it doesn't depend of the motion of the source, or your motion.

Light in a medium doesn't travel at c, but as Swansont said, is c/n.

Any speed less than c is not invariant.   In this case, the measured speed will depend on whether it is the observer or source that it moving relative to the medium.  But also as already mentioned, you need to use Relativistic velocity addition to get the correct answer.  Relativity doesn't "just go away" because light is now longer moving at c.

It is also important to understand that is the speed "c" that is important and not the speed of light itself.  When someone says that you cannot exceed the "speed of light", they mean "c", which is the speed light travels in a vacuum.

It is possible to exceed the speed of light for a given medium, even in that medium.   For instance, there is a type of nuclear reactor which is surrounded by water.  It produces subatomic particles which travel through the water at speeds greater than the speed of light through the water.  This produces a type of electromagnetic "shock wave", which is seen as a faint blue glow known as Cerenkov radiation.

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