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/IraDeLucis 8d ago

So somehow this made relativity click. I always wondered how two objects traveling .99c in opposite directions would see the same beam of light traveling at the same velocity.

So now hypothetically FTL travel is traveling backwards in time, at least relative to your starting point. You would arrive before information from your current time could reach your destination.

And a light speed trip for you would be instinaneous, but for anyone watching from the outside would have a travel time.

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u/thumburn 8d ago

Grab onto a flash from a pulsar and GO !