[adolph]

> light moves at a constant speed

While in the same medium, right?

The speed of light is a universal constant in a vacuum, like the vacuum of space. However, light can* slow down slightly when it passes through an absorbing medium, like water (225,000 kilometers per second = 140,000 miles per second) or glass (200,000 kilometers per second = 124,000 miles per second).*

https://www.space.com/15830-light-speed.html#section-speed-o...

[lxgr]

Light propagation in a medium is a quite different thing from light in vacuum.

For example, the speed of light in a medium is not the "speed limit" of things in the same medium, and particles in it can actually move faster than light: https://en.wikipedia.org/wiki/Cherenkov_radiation

In other words, the speed of light in vacuum plays a special role in both a vacuum and a medium.