If light travels at the same speed in all media, then in what terms is it slowed down or sped up in refraction?
In: Physics
Light doesn’t always travel the same speed. It only travels the same speed in all directions and in empty space. Light slows down plenty due to refraction.
I’ll try to ELI5 this, but it’s quite complicated. Light does effectively travel slower in different materials compared to a vacuum.
Light is an electromagnetic wave, its got an electric field and a magnetic field which oscillates. When light passes through a medium, this oscillating wave causes the electrons in the material to also oscillate – an oscillating electron in turn produces their own waves. This doesn’t happen perfectly in sync with the light wave, so the wave from the electrons is slightly behind the light wave. When you sum together all of these waves the “phase velocity” adds up to a slower speed than the speed of light in a vaccum. The confusing bit is the propagation of electromagnetic forces is still happening at the speed of light in a vaccum, but the time it takes for the wave to enter the object and leave the other side is longer than in a vacuum. Unfortunately I can’t think of a good analogy!
Things slow down when stuff is in their way. Like running through water is slower than running through air. Usain Bolt could probably run even faster in a vacuum, except for that pesky problem of needing oxygen to breathe (and an oxygen tank might outweigh the benefits of getting rid of drag by removing the air).
Light is a group of photons. Individual photons isn’t considered light. Photons themselves don’t slow down, but they cover larger distances when they pass through mediums (such as air, or glass) since they scatter at different angles. That’s what it means for light to “slow down”.
The speed of light *isn’t* the same in all media. When people refer to “the speed of light” they’re usually referring to the speed of light *in a vacuum*, which is the maximum speed at which information can travel. But light traveling through air or water or whatever will be slower than light moving through a vacuum. Usually not by much and the exact speed varies depending on the material, but slower nonetheless.
There’s also an interesting phenomenon called Cherenkov radiation, which occurs when a particle is moving through a medium faster than the speed of light in that medium. It happens in some nuclear reactors and causes a blue glow. A particle can *never* move faster than light in a vacuum, but in some mediums they can.
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