Light speed is light speed, and c is c. They coincide in a vacuum. Light always travels at light speed.
It's a little more complicated than that. Like, yes light is always traveling at c. But as it moves through a medium, it interacts with that medium. It gets refracted, absorbed, re-emitted, diffracted, etc: All things that add time to the photons' travels, lowering its observed velocity.
That is essentially what I was saying. The observed effect of light (the group velocity of light) travels at the speed of light. That is essentially to say, for everyday purposes like transmission of signals, the speed of light is the (observed) speed of light. In a vacuum, this is c, but in other media, the speed of light is slower than c, so c might be better referred to as the speed of causality.
Edit: I will mention that some peg c to the speed of light in any medium, and refer to the speed of light in a vacuum as c₀. This usage is endorsed in official SI literature.
It's a little more complicated than that. Like, yes light is always traveling at c. But as it moves through a medium, it interacts with that medium. It gets refracted, absorbed, re-emitted, diffracted, etc: All things that add time to the photons' travels, lowering its observed velocity.
That is essentially what I was saying. The observed effect of light (the group velocity of light) travels at the speed of light. That is essentially to say, for everyday purposes like transmission of signals, the speed of light is the (observed) speed of light. In a vacuum, this is c, but in other media, the speed of light is slower than c, so c might be better referred to as the speed of causality.
Edit: I will mention that some peg c to the speed of light in any medium, and refer to the speed of light in a vacuum as c₀. This usage is endorsed in official SI literature.
Ya I was just being a goof lol. Interesting about c vs c0 tho, never seen that usage before