Do electromagnetic waves really slow down?

Question

I have read somewhere that electromagnetic waves, when refracting only "appear" to slow down. That is when the wave goes thorough a material like glass it vibrates the electrons in the glass which generates its own electromagnetic wave that when added to the original wave makes it appear to slow down(i.e its crest moves slower in glass), however I believe the original electromagnetic wave does not slow down.

So if you add a light sensor on the other end of a glass and place a laser pointer at the other end. The speed of light record should be c is what I assume.

So I am wondering if the original electromagnetic wave ever slows down, or does it always only "appear" to slow down. I feel like an original electromagnetic wave should never slow down because the magnetic and electric fields should always travel at the speed of light everywhere, but I am not sure if this reasoning is correct.

edit: I understand that you could say that light slows down, if a photon is absorbed by an electron and then re-emitted again. However I was wondering if the original photon ever actually slows down on it's own, just like a ball might slow down due to friction just going on a path, traveling thorough a medium or if the same photon always travels at the speed of light unless it is absorbed by an electron?

For example in my original question about the laser and the light detector I wouldn't actually consider light to slow down if the photons that come out of the glass to the detector have somehow taken a non-straight path in the glass or are different photons than the ones entering the glass.

Answer 1

This question generates a lot, "you don't know what you're talking about" comments on the places where it is discussed on-line.

In my opinion, it doesn't matter which you chose.

First, there is the semantics problem. When somewhen says "the speed of light", it means the fundamental constant, $c$. When you're talking about index of refection, with Cherenkov, you talk about "charged particle moving fast than the speed of light"...in-the-medium is implied...at visible wavelength.

That will generate useless push back over semantics. There is no "speed of light" in a medium, it's a function of frequency. Gamma rays in water go $c$, visible light does not. Deal with it.

Now on to the actual physics question:

If you're working with Maxwell's equation in linear media, then the speed of the wave propagation is:

$$ v = \frac 1 {\sqrt{\mu_0\epsilon}} = c/n $$

That is a perfectly fine point of view.

The fact that $\epsilon \gt \epsilon_0$ means you can also view the polarized media as reradiating and EM wave with a phase shift that cancels the leading edge of the wave so that it only appears to propagate slower than $c$. If you want to do that: fine.

Now if, like me, you've detected individual Cherenkov photons: they're late. They don't show up at $t = L/c$. So that's fine, too. Same for pulses in co-ax cables.

Regarding the statement: "I feel like an original electromagnetic wave should never slow down because the magnetic and electric fields should always travel at the speed of light everywhere", I don't think that is fine.

Changing electric fields don't cause magnetic fields, and changing magnetic fields don't cause electric fields. Both fields are sourced by charges, currents, and their time derivative on the past light cone, as described by Jefimenko's equation (https://en.wikipedia.org/wiki/Jefimenko%27s_equations)--note, they are equivalent to Maxwell's equations.

With those equations: fields don't propagate; rather, the influence of charges and currents cause fields, and that cause propagates away from the source at $c$. The waves are then emergent phenomenon--they're not fundamental. (This is in-line with $c$ being the speed-of-causality).

So in this view: a light pulse propagating in media is caused by the original source and polarization sources in the media--all on the past light cone-to produce non-zero fields that move way from the source slower than $c$.

So: causality still moves at $c$, but the finite portion of the wave is slower, but the wave isn't something moving, it's an apparent motion of where there is not-zero field.

I don't think there is anything wrong with this view" you can have your cake (casualty moves at $c$), and eat it to: (but the non-zero field disturbance that looks like a wave or a photon, does not).