speed of light?

Question

Einstien relativity says that speed of light is a constant c in all frames.
But we know that light under goes diffraction when traveling thru some meduim, eg water, so then light travels slower!
Also in the Bose-Einstein condensate light has been slowed down to 38miles per hour !

So what did einstien really mean by saying speed of light is constant???

Answer 1

The speed of light is ALWAYS constant in all reference frames.
Here's the answer I gave recently to a related question:

The speed at which light APPEARS to travel depends on the medium it is traveling through. In a vacuum, light travels at c (299,792,458 meters per second). In any other medium, light APPEARS to travel at a speed less than c. This characteristic of light is what allows such things as lenses to function. All evidence I'm aware of still supports special relativity, indicating it is not possible for light (or anything else) to travel faster than c.

There have been several experiments in the past couple of years that have attempted to slow light down. The most recent one I'm aware of was reported in the Washington Post last month, which I've linked below. (Bear in mind it's a newspaper and not a scientific journal, so it's pretty slim on the specifics and might even be inaccurate in some areas.)

You may also want to check out Lene Hau's experiments with light passing through a Bose-Einstein condensate, which was discussed in the Harvard University Gazette (also linked below). In those experiments, light was "slowed" to around 17 meters per second.

When light interacts with a medium, it doesn't technically slow down as most people would think, but rather travels in one linear direction at that speed and only appears to slow due to the absorption and reemission of photons by the medium. This is explained in more detail in an article at Physlink, which I've also linked below. The discussion there may also answer some other questions you might have on the issue.

Answer 2

What Einstein meant was that the speed of light does not change depending on the motion or speed of the observer. If you were travelling at the speed of light and turned on a flash light, you might expect the light not to go anywhere, because you were already going lights top speed, but the light would still shoot away from you at the speed of light. Some might say then that would be the speed of light times 2 for an outside observer, but it wouldn't. If the observer was to calculate the speed of the light, he would also come up withe the same number, c. It has to do with time dilation as you approach the speed of light. Its too complicated to answer here. Read some more Einstein books.

Answer 3

Pluto was kicked out of the solar system - hint, hint.
Biochem has it right. I want to add that the speed of light in a vacuum is the fastest light can go.
If Pluto were right then rainbows wouldn't work. All colors of light travel at the same speed except when traversing the barrier between two dissimilar media. Here the wave nature of light is prominent.

Answer 4

This is common misconception. Light ALWAYS travels at the same speed. The reason for the differences in "speeds" is a difference in distances. When light goes through a medium is has to take a lot of extra turns to make it through the material. These extra turns make light take longer which we see as a slowing. Hopefully this makes sense.

Answer 5

I believe that Einstein was talking about the speed of light in a vacuum, not in different media. The speed of light in a vacuum is a constant.

Answer 6

The speed of light is always 'c' no matter what. It seems to slow down traveling through non-vacuums because the photons are constantly being absorbed and re-emitted, which slows down the wavefront but not the photons themselves.

Answer 7

I am going with biochem.

Answer 8

it is about 300000KM/S