Why does light propagate more slowly in glass?

Glass is a physical material composed of atoms, more specifically nuclear cores of silicon and oxygen surrounded by electrons. Light is a combination of electric and magnetic fields (see Maxwells Equations) and the more loosely-bound electrons move a bit in electric fields, absorbing a bit of the energy passing through. When the electrons move, they radiate energy, and in high quality glass essentially all the energy is reradiated, a few femtoseconds later. The overall effect is that the optical energy moving through glass is delayed somewhat by the vast number (about 1e22 per cubic centimeter) of outer electrons in all those atoms.

In vacuum, there are few or no electrons - a square-centimeter column between here and the Andromeda Galaxy has fewer electrons, and adds less additional delay than a centimeter thick panel of glass. The delay through one centimeter of glass is abouy 70 picoseconds, by the way. Science fiction author Samuel Delaney wrote stories about hypothetical "slow glass", with propagation velocities of centimeters per year; this fictional material would absorb light in summer daytime and re-radiate it on winter nights. However, in Real Life very little of the light would penetrate this magical glass, most of it would reflect, see Caveat 2 below.

Air is an interesting intermediate case, because there are enough atoms and electrons in air to slow light a wee bit. If the air gets denser, it slows more, and if air is somehow compressed to 10,000 atmospheres density, it becomes about as "refractive" as glass. Practically speaking, what this means is that air bends light a bit, because the air closer to the ground is more dense than higher up, so light rays curve a bit in earth's atmosphere. The light a few meters up moves slightly faster, so the optical wavefront bends. When the sun appears to set, for example, it is actually a few seconds below the horizon, the light is curving around the earth to get to you. Not very much, but some. If the earth's atmosphere was 4 times as dense, light would tend to curve all the way around it. Venus, with a far denser atmosphere, is an amusing case - if the air was more transparent (which it is not), it would look like the distant land is curling up into the sky, like you are at the bottom of a vast bowl. Reality is way more interesting than science fiction, but more difficult to describe in ancient languages like English.

And that is the best answer to questions about tensors in General Relativity. In some extreme circumstances, reality is exceedingly strange. Ordinary human language, well adapted to describing ordinary experience, is not up to the task. Mathematics is a recently developed artificial language for describing concepts that ordinary languages cannot. Tensor mathematics describes ultraextreme circumstances, like the regions around black holes, where humans and their machines will never go. Imagine describing a tree to an alien being who has never observed anything besides pebbles and sand, or pebbles and sand to an alien being residing in the plasma at the core of a star.

Caveats:

1) The above is only an approximation, and does not work if the glass is impure, thinner than a wavelength, or the light is short enough wavelength to ionize the glass atoms. Photon energy in electron volts is 1240 divided by wavelength in nanometers, blue light is about 400 nanometers and red light is around 700 nanometers, and electrons get ripped loose by short wavelength UV at around 5 electron volts (WAG) or or about 250 nanometers. That's why we use different materials than quartz and glass for lenses in UV imaging systems.

2) Because of mismatch in the dielectric constant, light coming straight at a flat glass plane reflects some of the energy - that is why you see reflections in the window at night. In daytime, what is outside is way too bright to see your own reflection in it. Our brains are extremely adaptive, and filter out uninformative phenomena like this, so it is difficult for neurotypical adults to "see" this. Ask a isolated tribesman who has never seen glass before about this.