Assume a block of transparent glass floating in space.
AIUI, light travels at the speed of light in a vacuum. (Stands to reason, doesn’t it?) When it passes through a medium, in this case, a block of transparent glass, though it could be an atmosphere or whatever, it slows down. The photon that enters the medium is not the same photon that comes out the other end. Instead, it is ‘absorbed’ (for want of a better word) by the glass molecule. This excites the electrons in the molecule, pushing them to a higher state. When the molecule returns to its normal state, it emits a photon. That photon goes to the next molecule, excites it, and then that molecule emits a photon, and so on. The last molecule in the medium emits a photon into the vacuum, which travels at the speed of light.
So light slows down in a medium because it has to do all of that exciting and returning to normal state and emitting photons. The photons that are emitted into the vacuum are emitted at the speed of light. Thus even though the beam is slowed down while it is in the medium, it ‘speeds back up’ when it exits – even though it’s not really speeding up because it’s not the same gang of photons.
Do I have that right? If not, please enlighten me.
Yes, you are understanding it correctly; however thinking about this phenomenon in terms of individual photons and atoms makes the situation overly complicated. To my mind, this is a problem with the way quantum theory is taught, with an overemphasis on photons (also known as energy eigenstates). The essential features of this interaction are better understood in a wave picture. The polarizability of the atoms gives the medium an index of refraction that slows down the passage of light waves.
While this story is often told, it contains significantly more lies than truth, and the truth is concentrated in a regime that the story teller usually isn’t interested in. The story has several immediate problems for explaining the refractive index that an everyday visible photon experiences in everyday transparent materials.
An atom or molecule that has been excited by a photon has no memory of the direction that photon was traveling in. The photon’s initial momentum imparted some momentum to the molecule, but that doesn’t affect the emission direction, which is isotropic. Materials that do what the story says are opaque.
A visible photon has a wavelength much larger than the size of an atom or molecule, maybe 10000 larger. The photon is not localized to a single atom and doesn’t bounce from one to the next in any sense. Its behavior must be determined by considering the collective properties of a hugh number of atoms at once.
The story talks about exciting atoms to higher energy levels. These energy levels are quantized, though. A photon at some random wavelength won’t be able to excite anything in the way that is suggested. There are consequences for refractive index for photons with wavelengths near absorption peaks, but these don’t come up in the simple situation at hand (e.g., visible light through glass).
The picture of a photon bouncing through a forest of molecules suggests that the index of refraction would scale with the number of molecules. While the index does scale with density, it’s (roughly) the electron density and not the molecular density that usually increases, yet the refractive index still goes up. For glass, for instance, the density is increased not by squishing SiO[sub]2[/sub] molecules closer together – they’re already packed tightly – but rather by introducing heavier elements into the mix like barium, magnesium, or lead.
So what does happen? It’s complicated. You need spatially distributed coherent behavior from a very large number of atoms, none of which are actually getting excited but all of which are experiencing some level of polarization/perturbation. One viable treatment (but not the only one) is to consider the molecules as arbitrarily small and in sufficient number that a (wavelength)[sup]3[/sup] volume can be treated as a continuum of harmonic oscillators coupled to the photon. Both the wave and particle aspects of photons are essential to any quantum treatment, though, so all classical metaphors break down.
(Pseudo-classical explanations of the interactions of photons and matter start making sense when you get into the x-ray region and beyond, where the behavior is governed by “photon hits atom” sorts of processes.)
Actually the process is more complicated than that: individual molecules have discrete absorption and emission spectra and if refraction were due simply to the absorption and re-emission of photons by electrons in molecules we would expect to see these discrete spectra in the refracted light, but we don’t.
I think the best answer may possibly be that the light that comes out is a superposition of the light that enters the glass and the light that is emitted by the glass due to the interaction with the incoming light and the bulk properties of the glass. Whilst the light that travel straight through the glass has a constant speed of c throughout the superposition does add a slight delay to the signal velocity.
I’m willing to be shown to be wrong though.
edited to add: this was not a response to Pasta, it’s a response to the OP and those agreeing with the view.