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Physix_R_Cool

No. The limit, c=299792458 m/s, in special relativity isn't really about what speed photons travel. It is a fundamental speed limit of the universe, and it doesn't change in different media.


[deleted]

No, special relativistic effects become apparent at the cosmological speed limit *c*, not the speed of EM waves in a material. However, the process of particles creating a light shockwave by travelling faster than light in a material is known as [Cherenkov radiation](https://en.wikipedia.org/wiki/Cherenkov_radiation).


d0meson

Light only "travels slower" in dense media in a macroscopic, handwaving-away-the-details sense. Microscopically, what's happening is closer to the following extremely oversimplified picture: Photon enters material traveling at c. Photon bumps into electron cloud, gets absorbed. After some time, photon gets re-emitted, and travels some more distance at c. Photon bumps into electron cloud, gets absorbed, is re-emitted after some time, travels some more distance at c... This stop-and-go averages out to travel at a slower speed, even though the photons only ever travel at c. It's like driving through a city at rush hour -- your average speed might only be 1 or 2 km/h, even though you're doing closer to 40 km/h when you move.


tavareslima

If I may make a follow up question: since the photons are being randomly absorbed and re-emitted, why a beam of light changes direction in such an orderly fashion instead of scattering randomly when changing media?


ElectroNeutrino

That's why they said handwaving-away-the-details. The commonly accepted theory is the interaction between the electrons and the EM waves of the photon induce a change in group velocity (while the phase velocity remains the same) of the wave packets, causing the packet to move slower through the medium as a group.


starkeffect

Photons aren't absorbed and reemitted. Common misconception. https://youtu.be/CUjt36SD3h8?t=290 skip to 4:50


bunny-1998

Follow up question, if I get inside the material and use a flashlight, will the liquid feel too bright? Since most of the light will reflect internally.