this post was submitted on 19 Nov 2023
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[–] ziggurism@lemmy.world 16 points 2 years ago (15 children)

The fact that light cannot change speed is one of the core axioms of relativity

[–] trash80@lemmy.dbzer0.com 5 points 2 years ago (14 children)

Light doesn't travel the same speed in water or glass as in a vacuum.

In a medium, light usually does not propagate at a speed equal to c; further, different types of light wave will travel at different speeds. The speed at which the individual crests and troughs of a plane wave (a wave filling the whole space, with only one frequency) propagate is called the phase velocity vp. A physical signal with a finite extent (a pulse of light) travels at a different speed. The overall envelope of the pulse travels at the group velocity vg, and its earliest part travels at the front velocity vf.

https://en.wikipedia.org/wiki/Speed_of_light#In_a_medium

[–] Neato@kbin.social 6 points 2 years ago (7 children)

That's light as an aggregate wave. Photons, actual light, always travel at c. What's happening in a medium is the rapid absorption and readmission of photons. The probability of admission is based on structure of material causing things like lens or mirrors to work.

You can think of it as the photons having to jump between platforms before the can continue running at c.

[–] Entropius@lemmy.world 3 points 2 years ago (1 children)

What's happening in a medium is the rapid absorption and readmission of photons. […]

You can think of it as the photons having to jump between platforms before the can continue running at c.

That’s an intuitive model, but unfortunately it doesn’t have the advantage of actually being correct. Photons are not being absorbed and reemitted. See here for why: https://lemmy.world/comment/5444224

[–] Neato@kbin.social 0 points 2 years ago (1 children)

That is wrong. Stochastic yes. Photons emission is probabilistic. Destructive interference causes emission to overwhelming follow classical wave theory. Here's a better explanation with a neat graphic.

https://physics.stackexchange.com/questions/466/what-is-the-mechanism-behind-the-slowdown-of-light-photons-in-a-transparent-medi

[–] Entropius@lemmy.world 2 points 2 years ago

It sounds like you’re conflating different concepts. A stochastic process like absorption/reemission would blur the light, so that’s not it. And the linked explanation is basically correct (in classical physics at least), but it doesn’t corroborate what you originally claimed as that’s not necessarily requiring absorbing anything. Photons can jiggle the charged particles in glass and get them to make new phase shifted light despite not being absorbed.

https://youtu.be/YW8KuMtVpug

https://youtu.be/CiHN0ZWE5bk

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