this post was submitted on 19 Nov 2023
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submitted 2 years ago* (last edited 2 years ago) by [email protected] to c/[email protected]
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[–] [email protected] 1 points 2 years ago (2 children)

They change direction and speed, right?

[–] [email protected] 15 points 2 years ago (1 children)

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

[–] [email protected] 6 points 2 years ago (2 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

[–] [email protected] 5 points 2 years ago (2 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.

[–] [email protected] 4 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

[–] [email protected] 2 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

[–] [email protected] 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

[–] [email protected] 2 points 2 years ago (2 children)

Now I'm not sure how reflective telescopes work.

[–] [email protected] 2 points 2 years ago

reject reflectors return to long tubes

[–] [email protected] 1 points 2 years ago (1 children)

Interference in matters structure causes classical wave like behavior.

[–] [email protected] 1 points 2 years ago

I find so much of physics to be very intuitive and then you have light.

[–] [email protected] 4 points 2 years ago* (last edited 2 years ago) (2 children)

But doesn’t relativity explicitly state that c is the speed of light in a vacuum, and travelling through other mediums explicitly changes and is explained by relativity?

I am 100% a layman and do not know the answer.

[–] [email protected] 3 points 2 years ago (2 children)

Not really no. Special relativity explains the relationship between space and time. General relativity expands on this to account for gravitation.

One of the postulates (i.e. assumptions) of relativity is that the speed of light in vacuum is the same for all observers. But the theory doesn't actually require any particular value for c, it only needs it to be constant. And it doesn't explain the behavior of light in a medium at all.

In fact, relativity doesn't explain the mechanism by which light interacts at all, that is the domain of Quantum Electro Dynamics.

[–] [email protected] 2 points 2 years ago

Wow that is so interesting. So am I understanding that relativity explains space, time and gravity’s interactions with one another, while quantum science explains interactions with much smaller objects like matter?

[–] [email protected] 2 points 2 years ago

the speed of light expressed in units of distance per time, is a dimensionful quantity so it probably doesn’t mean anything to say some theory does or does not predict a value for it. The value is entirely determined by how big you choose your yardsticks and sundials to be, which is arbitrary convention.

It is only meaningful to talk about theoretical predictions of the values of constants if they are dimensionless, like the fine structure constant.

However relativity does suggest as a natural point of view that space and time are just orthogonal directions in a unified spacetime. In this point of view, relativity gives you the option of measuring your timelike and spacelike coordinates with the same yardstick (which you may still choose arbitrarily). And then relativity does predict its value. It’s 1. No units.

[–] [email protected] 1 points 2 years ago (1 children)

I don't know. I thought I used to know.

[–] [email protected] 4 points 2 years ago

This is how I feel every time I touch any non-basal physics topic.

I swear this made sense once upon a time....

[–] [email protected] 3 points 2 years ago (1 children)

No, they don't. They can get absorbed and re-emitted, and the space they are moving though can compress sideways. But they can't make curves at all.

[–] [email protected] 1 points 2 years ago (2 children)

Do lenses absorb and re-emit light?

[–] [email protected] 3 points 2 years ago

That's basically all that refraction is. A dead giveaway is that light doesn't move at the speed of light in them.

[–] [email protected] 2 points 2 years ago* (last edited 2 years ago)

Yes.

Don't think about individual photons. Think about billions of them with destructive and constructive interference. The probabilities of all the sitting l additive waves of light.