Just doing a little recreational reading and came across this really interesting piece on light. Funny,never even touched this stuff back in college. Have read many articles and books about quantum mechanics, particle physics, string, and brane theories, etc... but never seen this perspective......
Things can travel faster than light; and light doesn’t always travel very fast.
The speed of light in a vacuum is a constant: 300,000km a second. However, light does not always travel through a vacuum. In water, for example, photons travel at around three-quarters that speed.
In nuclear reactors, some particles are forced up to very high speeds, often within a fraction of the speed of light. If they are passing through an insulating medium that slows light down, they can actually travel faster than the light around them.
When this happens, they cause a blue glow, known as “Cherenkov radiation â€, which is (sort of) comparable to a sonic boom but with light. This is why nuclear reactors glow in the dark.
Incidentally, the slowest light has ever been recorded travelling was 17 meters per second – about 38 miles an hour – through rubidium cooled to almost absolute zero, when it forms a strange state of matter called a Bose-Einstein condensate.
Light has also been brought to a complete stop in the same fashion, but since that wasn't moving at all, we didn't feel we could describe that as "the slowest it has been recorded travelling".
Things can travel faster than light; and light doesn’t always travel very fast.
The speed of light in a vacuum is a constant: 300,000km a second. However, light does not always travel through a vacuum. In water, for example, photons travel at around three-quarters that speed.
In nuclear reactors, some particles are forced up to very high speeds, often within a fraction of the speed of light. If they are passing through an insulating medium that slows light down, they can actually travel faster than the light around them.
When this happens, they cause a blue glow, known as “Cherenkov radiation â€, which is (sort of) comparable to a sonic boom but with light. This is why nuclear reactors glow in the dark.
Incidentally, the slowest light has ever been recorded travelling was 17 meters per second – about 38 miles an hour – through rubidium cooled to almost absolute zero, when it forms a strange state of matter called a Bose-Einstein condensate.
Light has also been brought to a complete stop in the same fashion, but since that wasn't moving at all, we didn't feel we could describe that as "the slowest it has been recorded travelling".
:learn:
its got cartoons to help you along. 
