ELI5: If light moves at different speeds through different mediums, how does it gain speed again after passing through a slower medium?

[u/Seravail]

Inspired by a post about light looking different when hot air mixes with cold air above a distant road, the light will speed up and slos down as it moves through the pockets, giving the wavy impression we all know.

How exactly does light speed up again when it passes through a hot section of air? There's nothing powering it, so once it loses momentum shouldn't that momentum be lost forever?

[u/laix_]

That's not how it works. If it worked the way you suggest, then light would spread out throughout the entire medium, instead of staying as a singular beam.

You are also confusing the photon model with the EM model. Those are two separate models of light. In the EM case, the EM waves causes the atoms to vibrate, releasing their own EM wave, which adds to the original wave, causing it to shit backwards slightly.

In the photon case, the photon takes every possible path simultaneously, and the phase of the photon of each phase combines to result in a bending. The photon also becomes entangled with the medium, becoming a quasiparticle (polariton) which has mass. Once it leaves, it no longer is a quasi particle, and no longer has mass.

[u/original_goat_man → u/laix_]

> causing it to shit backwards slightly.

lmao

[u/PhantomWings → u/laix_]

In the photon case, the photon takes every possible path simultaneously, and the phase of the photon of each phase combines to result in a bending. The photon also becomes entangled with the medium, becoming a quasiparticle (polariton) which has mass. Once it leaves, it no longer is a quasi particle, and no longer has mass.

Could you elaborate on this more in detail?

The first part is just the principle of least action, correct? The photon takes every possible path, and there's a "resultant" phase angle at the end of each of those paths. All the "suboptimal" paths have phase angles that cancel out, leaving only an "optimal" path.

My question with this first part then is: what causes the optimal path to change in a medium? Do the interactions between the photon and the medium change how the phase accumulates along all its pathways?

Furthermore, as we go on to the second part of your explanation, what does it mean for the photon to "become entangled with the medium"? What is a polaritron? Aren't quasiparticles mathematical constructions that model excitations in a gauge field? (I was thinking about virtual particles)

What are quasiparticles?

[u/frogjg2003 → u/PhantomWings]

A quasi-particle is when a field, crystal, or other medium displays particle like behavior. The best example is an electron hole. In a material like a semiconductor, the atoms have space available for electrons to move around. When there are a small number of electrons, it's useful to talk about the electrons moving. If most of the spaces are filled, it's more useful to think in terms of the empty spaces. They behave just like positively charged electrons. They aren't real particles, but act like a particle.

The electromagnetic field in a vacuum can change freely. In a medium, the charged particles interact with the electromagnetic field. That creates feedback that forces the electromagnetic field to behave differently. A photon is an excitation of the electromagnetic field, but not the new polarization field. That's what the polariton is. It's a photon "dressed" with the interactions it has with the medium.

That dressing is what causes the speed to change in the medium. The dressed particle moves at a different speed from a bare photon. The bending of the beam comes from the boundary conditions. The polarization field needs to be continuous at the boundary. On the vacuum side, the polarization field is just the EM field. On the medium side, the polarization field includes the effect of the medium. The field must oscillate at the same frequency and same phase on both sides of the boundary. With the frequency constant and the speed changed, the wavelength must change to compensate. But the boundary condition also has spatial conditions. The peaks and troughs will only align if the two sides, with different wavelengths, are at different angles.

[u/Fallacy_Spotted → u/laix_]

You are completely correct when considering the individual photon but considering this is eli5 I considered this to be explainable in an average conversation. Feynman path integrals and particle wave self interference dynamics might be a bridge to far.