Light travels though unoccupied space at 1c (empty space; a vacuum).<quoted text>
A non-vacuum media...
Light *only* travels through unoccupied space, and *always* at 1c.
Don't stop reading, RR.
When a photon encounters a particle of matter, it gets absorbed by the particle.
Shortly thereafter, a new photon gets emitted from the particle.
There's a brief duration of time between absorption and emission. It's a very, very brief duration... but, it is quantifiable.
The new photon travels at a speed of 1c until it encounters another particle of matter, where and when it gets absorbed and a new-new photon is re-emitted.
The processes is repeated, over and over, as light traverses what we're referring to as occupied space... which really consists of mostly empty space.
Have you ever heard that an atom consists of mostly empty space? It's true.
Molecules consists of atoms. Thus, molecules are mostly empty space.
The stuff that occupies space consists of molecules. Thus, the stuff consists of mostly empty space.
Light's speed is constant, at 1c, when it's traveling. Otherwise, it's waiting to be emitted.
The overall effect is that light's apparent speed varies. The variability depends upon the duration of time between absorption/emission of photons when they encounter matter... and the distance they travel between encounters.
Thus, it's perfectly acceptable to posit that light's speed is constant.