X-rays "faster" than the speed of light, and more refractive index questions
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X-rays "faster" than the speed of light, and more refractive index questions
Optics puzzles part 4
I made a follow-up to the last video about the index of refraction, answering a few questions people had about the topic. Why does light slowing down imply that it will bend (if you want something better than the angled-car-into-mud analogy)? What’s going on with birefringence? What’s the end of the barber pole explanation? And, most fun of all, how is it possible for this refractive index to be lower than 1?
This is a great video! However it is also true that the GROUP velocity (not the phase velocity) can be faster than c. This phenomenon is common in nonlinear media or media with gain, where the front of a wavepacket can get amplified and the back can get attenuated, producing an effective group velocity > c. I am not sure whether there is a conservation law there, like when vp > c, vg< c, but I don’t see why there would be.
In any case, in optics we actually define a third velocity, signal velocity, which REALLY can’t be faster than c. I’ve never actually seen someone calculate this in practice though.