Light Speed in Vacuum vs Media: A Question

[HyperUniverse]

Hi,

I know the speed of light is the highest in vacuum.
And in any other mediums is a bit slower.

I just want to know...
...if a light passes through a medium (water let's say) and comes out in vacuum, will it retain the same speed as it had in the water, or will it increase back to its maximum speed?


Thanks.

 

[Doc Al]

The light will have its usual (maximum) speed in a vacuum, regardless of where it came from.

 

[HyperUniverse]

Could you be a bit more explicit, please?
Has anybody done such tests? Where can I read more about?

Because I take it like this...let's imagine that we shoot an arrow through the air.
Then the arrow passes through "a floating ball of water" (just imagine it).
Then it comes out of this water, back into the air.

Obviously, when it passed through the water its speed was slower, but when it came out of the water and back into the air, it did not increase its speed back to what it was before entering the water...


So how the light will increase its speed back to maximum when leaving the water?

Thanks

 

[Doc Al]

So how the light will increase its speed back to maximum when leaving the water?

Think of light as a wave, not as an object like an arrow. The speed of a wave depends only on the characteristics of the media its traveling through at any given moment, not on where it's been. In that respect, light acts like any other wave. But for light, the maximum speed is attained when traveling through a vacuum.

 

[Ryan_m_b]

Does light ever "slow down" or is it merely absorbed and re-emitted by the atoms of the media it is passing through with the time this takes slowing down the overall "speed"?

 

[BruceW]

The reason light is slower in a medium is not because it is being absorbed and re-emitted.
It is slower simply because of the presence of ions.

 

[Ryan_m_b]

The reason light is slower in a medium is not because it is being absorbed and re-emitted.
It is slower simply because of the presence of ions.

How would ions slow down light? The implication is that a bulk material with no ions would allow light to travel at the same speed as in a vacuum, this doesn't seem correct.

 

[BruceW]

In a plasma, the EM wave gets slowed down because of the presence of ions. I guess I assumed that it would be a similar reason for the slowing of an EM wave in normal matter.
But maybe it is slowed down because each of the atoms act slightly like a dipole (which would require a quantum-mechanical treatment?)