I am in complete agreement with you, R—just adding some more physics.

The speed of light in vacuum, c, is constant and the top speed limit. Light in a physical medium slows down in proportion to the material’s refractive index n (its optical “density”), which is why it gets bent as it passes through. Light entering a pane of glass at an angle gets bent toward the normal on entering the glass by effectively slowing it down as it bounces around (still at speed c) from atom to atom within the glass with a refractive index n=1.46.
(Definition: the normal is the perpendicular to the surface. It’s from where we measure angles of incidence, reflection, and refraction.)
On exiting the other side of the glass pane, it bends back away from the normal to be parallel to its path into the glass, and it continues at speed c with nothing impeding it. Or close to c. Vacuum has a refractive index n=1, air has n=1.0003.
Water (n=1.33) has a lower refractive index than glass, so the light isn’t bent as much at the surface or slowed as much by bouncing around between water molecules. Diamond is extremely dense optically, n=2.4

If you ever looked inside a water-cooled nuclear reactor (I have done so) you’d see a blue-green glow called Cherenkov radiation. It comes from nuclear particles that are traveling faster within the water than light travels (in its effective bouncing speed). It is the optical equivalent of a sonic boom, from an object traveling faster than light (or sound) than the speed of light (or sound) in the dense medium. Again, outside of a dense material (air, water, glass, diamond), light goes at speed c, faster than any material object can travel.

Absolutely fascinating - all of it! Thanks, Rr!!!