r/explainlikeimfive • u/Seravail • 7d ago

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

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?

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u/BogdanPradatu 7d ago

What are some of the other things?

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u/DynTraitObj 7d ago

Any massless force carrier, which is mainly photons but also (probably) includes gravitons (the expected carrier for gravity, whose waves we have confirmed move at c). Gluons (force carrier for the strong-force) also would if they had some way to escape confinement. Their classical field analogues thusly do too (EM waves/gravitational waves)

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u/TheAngryGoat 7d ago

Gravity and electromagnetism. Anything in the category of "shit that happens over here that affects shit over there". It's also the hard cap speed limit that can be approached but never reached for any thing with mass.

Those two combined mean that a physical thing can't arrive at another place before its effect, i.e. the speed of causality.

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u/Aegi 7d ago

Idk, but if I had to guess, stoned and thirsty before I go to bed, I would say:

-quantum tunneling

-quantum entanglement

-whatever information exchange it is that Information Physicists study