The constant c is the speed of light in a vacuum. Different mediums have different speeds of light within them; in fact, it's actually possible for light to not be the fastest thing in a given medium (for more precise definitions of all those words): https://en.wikipedia.org/wiki/Cherenkov_radiation

When you refract light, you change its speed, slowing it down.

https://en.wikipedia.org/wiki/Refractive_index#:~:text=The%2....

https://www.zmescience.com/science/physics/how-to-slow-trap-...

Altought the answers so far posted are not exactly incorrect, it is also important to note that the geometry (that is, a cylinder for optical fibers) of the waveguide used influences the group velocity of the propagating electromagnetic field.

This will always end up being slower than a plane wave in the same material, usually only somewhere between 10%-90% (and this also depends on the wavelength) of the free-space group velocity.

In a vacuum. See Cherenkov radiation[0].

[0] https://en.wikipedia.org/wiki/Cherenkov_radiation

c is the speed of light in a vacuum

Downvotes are unfair; this part of physics I either forgot, or never learnt, I can't tell. Thanks to those who pointed me at links.