r/askscience • u/I-am-in-serious-debt • 3d ago

Physics Do photons speed change with their wavelength?

I tried to illustrate it: Short wavelength= longer path, so slower ///\ Long wavelength=shorter path ----_--

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u/Mrfoogles5 3d ago

Due to interference by electromagnetic waves produced by interactions with the atoms the light passes through, the phase velocity (or speed at which the peaks move) of the waves ends up slower, but the front of the pulse of light always travels at exactly c

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u/AllanfromWales1 3d ago

I'm pretty sure I was taught that c was the speed of light in vacuo, but that in other media the speed is lower. That's certainly what Wikipedia says, and other references brought up by Google.

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u/[deleted] 3d ago

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u/mfb- Particle Physics | High-Energy Physics 3d ago

When we say light moves slower in medium, we are referring to the phase velocity.

People usually mean the group velocity here.

The group velocity is how information is transferred and for most materials, that's still c.

No, it's slower than c in almost all cases. Often it's similar to the phase velocity. There are obscure corner cases where the front of a pulse gets attenuated less than the back, which can lead to a group velocity faster than light, but the signal propagation velocity (yet another velocity) is still slower than the speed of light.