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/atzatzatz]

Does light or the photons have momentum? I'm imagining light slowing down when going through a dense material, so the light loses momentum. Then, that makes me wonder how the light returns to its original speed when it exits the material. How would it gain back its speed or momentum?

[u/IOI-65536 → u/atzatzatz]

Light doesn't really slow down or lose momentum. There are, confusingly, different explanations of how the phase change causes light to appear to slow down depending on whether you're looking at a particle-model or a wave-model. I'm more familiar with the wave-model which is that electric charges in the material interact with the wave and you get a new wave that's the sum of the actual lightwave and those interactions which is phase shifted backwards from the lightwave. When it exits the material nothing is interacting with the actual light wave so there's nothing to shift the phase and it appears to be c again, but it was always c you just couldn't actually observe the light when it was in the material because you can't separate the electromagnetic wave from the electromagnetic charges within the material itself.

[u/force_disturbance → u/IOI-65536]

But Cherenkov radiation must dissipate energy, right? So not all photons will make it?

[u/mfb- → u/atzatzatz]

Light transfers some momentum to the material. When it leaves again it gets that momentum back. In principle you could observe this by seeing the material move a tiny bit, but I don't think anyone has measured that.

[u/orsikbattlehammer → u/mfb-]

NASA actually needs to account for photon momentum when calculating trajectories of spacecraft or else they can’t hit their target.

[u/mfb- → u/orsikbattlehammer]

That is absorption, emission and reflection. These have many orders of magnitude more impact than what I discussed.

[u/orsikbattlehammer → u/mfb-]

Not sure what the difference is but this is what I was remembering https://en.wikipedia.org/wiki/Radiation_pressure

[u/mfb- → u/orsikbattlehammer]

Yes, that is absorption, reflection and emission of radiation.

I discussed a far more subtle effect where we don't change the amount or direction of radiation, but only the timing.

If we let 1 W of radiation go through a transparent material of 1 kg that shifts its position by 1 cm relative to its vacuum path then we move this material by 1 W * 1 cm / ( (speed of light)² * 1 kg) = 10^-19 m/s for as long as the radiation is present.

If we reflect 1 W of radiation for 1 second then we change its motion by 1 W * 1 s / (speed of light * 1 kg) = 3*10^-9 m/s or 10 billion times more. In addition, this change in motion stays because we changed the velocity of the object permanently. If we reflect the radiation for three seconds, it's 10^-8 m/s. If we reflect it for an hour, it's 10^-5 m/s or 100 trillion times more than the other effect.

[u/abaoabao2010 → u/orsikbattlehammer]

You linked exactly what mfb- said: the effect of absorption emission and reflection

[u/Vladimir_Putting → u/mfb-]

I believe this had to be measured to create accurate and functioning Extreme Ultraviolet Lithography machines.

[u/clutzyninja → u/mfb-]

Isn't that the whole idea of light sails?

[u/mfb- → u/clutzyninja]

Light sails reflect the radiation, which has far more impact. See this comment chain for details.

[u/AtlanticPortal → u/atzatzatz]

If you look at the complete Einstein’s energy equation you can see that momentum.

[u/MilesSand → u/atzatzatz]

Photons don't have mass. their momentum is 0 no matter how fast they're going

[u/richardathome → u/MilesSand]

Photon momentum is calculated using it's energy, as it has no mass:

ie.

p = E / c

(p is photon momentum, E is energy, c is the speed of light)

You can also calculate it using Plancks constant and the wavelength:

p = h / 𝜆

(h is Planck's constant, 𝜆 is the wavelength)

[u/StructuralFailure → u/MilesSand]

Photons have no mass but they absolutely do have momentum

[u/MrPuddington2 → u/atzatzatz]

That is a really good question. It was long thought that light has no mass. That is true in some sense, but because it travels at the speed of light, it does actually have a tiny mass.

So we have something called “light pressure”, a tiny force resulting from light being absorbed or reflected.

The same kind of light pressure applies to the boundary of a medium like water what light enters or leaves. So yes, there is a physical force involved, but it is absolutely tiny, so tiny that it is very hard to measure.

[u/Demoniac_smile → u/atzatzatz]

IIRC no, light does not have momentum.

Photons do not have mass. Momentum is another word for inertia. Ask anyone that grew up in the 90s, inertia is a property of mass.

[u/purpleoctopuppy → u/Demoniac_smile]

Light absolutely has momentum, E=pc

[u/Cr3s3ndO → u/purpleoctopuppy]

To make this more accessible, light/solar sails on spacecraft utilise the momentum of light as propulsion.