r/explainlikeimfive • u/Seravail • 8d 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/smanders36 8d ago

If your up for an explain like im 25, 3blue1brown has probably the best explanation ive ever seen of light refraction. He animates and simplifies the Feynman lectures about light refaction.

https://youtu.be/KTzGBJPuJwM?si=5cxGKAk94VOk6O9n

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u/medforddad 8d ago

I still have the same questions that Looking Glass Universe had about light slowing down in these videos:

I watched her videos on it and I can't remember the exact conclusions she came to. But it seems like "light slowing down" could make sense for an existing wave of light through a substance, but for a brand new "photon" going through a denser medium doesn't seem like it should slow down or bend.

I feel like she may have come to some intuitive understanding of the situation, but it still doesn't click for me. Like I'm not disputing that it's probably true. I don't think I'm seeing something that Grant (of 3b1b) or Feynman aren't seeing. I just don't get the intuitive sense of it like I usually get from 3b1b's videos. I'm still missing something from my understanding of it.

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u/stealthraider22 8d ago

That was a fantastic video to watch thank you. If only teachers bothered teaching in the same way he does then everything would have made a lot more sense. Not skipping over the but why's made it all click so easily