Speed of Light: Does It Change?

[Lunct]

Does the speed of light change in different substances, or does it seem to change because it gets absorbed and emitted by all the particles?

 

[Wrichik Basu]

It changes. In media other than vaccum, the speed of light is decreased, and the magnitude of the new velocity depends on the refractive index of the medium.

 

[Ibix]

Depends what you mean. Light interacting with matter is complicated. Inside a material, there are electric fields from the electrons and nuclei, so Maxwell's equations don't simplify to the wave equation for a wave traveling at c. The speed of light changes; it isn't random walk absorption and re-emission.

That doesn't really mean anything in relativistic terms. In relativity, the fact that light in a vacuum travels at c is more or less irrelevant - c is actually just a natural scale factor between units of time and distance. That doesn't change, whether light is locally moving at that speed or not.

 

[DrGreg]

We have an FAQ about this: Do Photons Move Slower in a Solid Medium?