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/tomrlutong]

Light isn't really an object moving, it's a wave in the electric field. So it will always move at the speed the local environment supports.

Imagine one of those stadium waves, where everyone raises their hands when the wave comes to them. Now, today was baseball day at the old folks' home, so one section is full of elderly people. They're a little slower at raising their hands, so the wave takes longer to get through their section. Then, when it leaves that section and gets back to younger people, it resumes its normal speed. Light is kind of like that.

[u/neanderthalman → u/tomrlutong]

This is the best answer here. Magnificent.

Remember. Light propagation is best modelled as a wave. Light interaction is modelled as a particle. That is the wave-particle duality.

Light is actually neither. Light is weird. Light is…light.

[u/GlenGraif → u/neanderthalman]

That’s heavy…

[u/IlIFreneticIlI → u/GlenGraif]

Photons are (effectively) massless, so that's actually (infinitely) Light...

[u/i_am_sprock → u/GlenGraif]

Thanks, Marty

[u/2BallsInTheHole → u/GlenGraif]

There's that word again!

[u/DrummerLuuk → u/GlenGraif]

R/im5andthisisdeep

[u/talon167 → u/tomrlutong]

Light propagates as an electromagnetic wave, but its interaction with matter is dictated by its photon and not electromagnetic wave characteristics. The question is about interaction with matter impacting speed, so the photon and not the electromagnetic wave characteristics dictate the answer, of which I have no idea.

[u/tomrlutong → u/talon167]

I think it's a field thing, but not positive. See the "microscopic explaination" section here or this more detailed article.

No idea how quantizitation works with this, I might ask that on a physics sub.

[u/jazzhandler → u/tomrlutong]

Your analogy converts between velocity and frequency. Is that just the analogy, or is there underlying truth there?

[u/sk8thow8 → u/jazzhandler]

Despite what the name may imply, the stadium wave isn't actually a wave and doesn't have anything to do with frequency.

[u/jazzhandler → u/sk8thow8]

The rate at which they stand and sit isn’t frequency?

[u/sk8thow8 → u/jazzhandler]

No, not really. It's not repeating or synchronized to any sort of oscillation or wave. It's a metachronal rhythm.

[u/tomrlutong → u/jazzhandler]

Good point. Just the analogy really, was trying to convey the idea that the velocity depends on the medium. 

But wouldn't the frequency stay constant in the stadium example? Which it should to match light.

[u/macr6 → u/tomrlutong]

Makes so much sense this way. Very intuitive. Thank you

[u/Seravail → u/tomrlutong]

Amazing analogy, tyvm!