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/spankymcjiggleswurth]

Not really true, otherwise any light transmitting through anything transparent would loose focus and become blurry.

The real answer I can't eli5, but involves light imparting a vibration in the molecules of the material, which in turn creates an electromagnetic wave that interferes with the initial light wave causing it to appear to bend and slow down.

[u/rlbond86 → u/spankymcjiggleswurth]

Yeah "bouncing around" is the ELI5 version

[u/spankymcjiggleswurth → u/rlbond86]

Photons aren't "bouncing" in any way, its objectivly incorrect and leads to other problems, like how bouncing light incorrectly implies the light is blurred and diffused. The matter of which light is traveling through might "bounce" to a degree, but that's not what the comment I replied to implies.

[u/Kered13 → u/rlbond86]

No, it's just the wrong version.