Comment on Speediest little fella.

• ziggurism@lemmy.world ⁨10⁩ ⁨months⁩ ago

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

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  • trash80@lemmy.dbzer0.com ⁨10⁩ ⁨months⁩ ago

    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.

    en.wikipedia.org/wiki/Speed_of_light#In_a_medium

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    • Neato@kbin.social ⁨10⁩ ⁨months⁩ ago

      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.

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      • Entropius@lemmy.world ⁨10⁩ ⁨months⁩ ago

        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: lemmy.world/comment/5444224

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      • trash80@lemmy.dbzer0.com ⁨10⁩ ⁨months⁩ ago

        Now I’m not sure how reflective telescopes work.

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    • there1snospoon@ttrpg.network ⁨10⁩ ⁨months⁩ ago

      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?

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      • sushibowl@feddit.nl ⁨10⁩ ⁨months⁩ ago

        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.

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      • trash80@lemmy.dbzer0.com ⁨10⁩ ⁨months⁩ ago

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

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• marcos@lemmy.world ⁨10⁩ ⁨months⁩ ago

  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.

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  • trash80@lemmy.dbzer0.com ⁨10⁩ ⁨months⁩ ago

    Do lenses absorb and re-emit light?

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    • marcos@lemmy.world ⁨10⁩ ⁨months⁩ ago

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

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    • Neato@kbin.social ⁨10⁩ ⁨months⁩ ago

      Yes.

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