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I don't understand why this does not imply the possibility of faster than light communication. If you had a long tube with a zero refractive index, couldn't you type Morse code into one end, by simply turning the signal on and off, and have the receiver receive the message immediately?

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3blue1brown's video covers this. The effect of "changing speed" is a change to the light's wavelength, not the speed of an impulse through the material--imagine taking a standing wave and pushing the peaks and valleys up and down so the wavelength looks really long. And since the frequency is unchanged, there's your appearance of high speed. This happens because the material changes the phase of the wave continuously.

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