Why does light slow down?

It’s well known that the ‘universal speed limit’ of c is the speed of light in a complete vacuum. We also know that light slows down when it enters something. Recent experiments have even slowed light to moderate bicycling speeds.

I understand (to some extent) why light refracts, and from the wave point of view the math works out. What I’m wondering is if we have any idea of what the mechanism that slows down the photons is.

Until recently, I was under the (apparently mistaken) impression that the photons never slowed down, they were just absorbed and emitted by matter, requiring more time to get through. So can someone clear up my ignorance, or is this something we don’t know for sure?

The professor in my college optics class left me with this same impression, but he didn’t seem too sure of it. This is also the impression I got from reading Richard Feynman’s QED, but I have since read things that contradict the idea. In short, I don’t know, but I’m as curious as you to find out.

Not that you could ride a bicycle through a Bose-Einstein condensate! :smiley:

Good OP overall. I’ll have to check my physics textbooks. I assume it’s a EM-wave function type thing. (how’s that for a useless response?)

I don’t see how that could be the case. If a laser passes through a block of glass, it is still coherent on the other side. How could it stay coherent if all the photons have to be absorbed and re-emitted? And yes, they would all have to be absorbed and re-emitted if the speed of the overall beam is to be reduced.

There is scattering, a phenomenon in which photons bounce off of air molecules, resulting in our blue sky effect. But this isn’t exactly absorption/emission, it’s more like pinball.

What does it mean to ‘absorb’ a photon? In a gas or liquid, the molecules are not bound like a spring, so it is not possible to absorb a photon by exciting an oscillation mode of the entire molecule. Similarly, it is not feasible to be exciting atoms or you’d have ionization going on (that’s why UV light is not transmitted by the ozone layer, for example.) If you were exciting atoms you’d get spectral lines. Of course, you could excite oscillations in molecules (spring-like vibrations within a molecule) but I still think the energy required is not usually in the visible spectrum.

I’m also not clear what it would mean for an air molecule to ‘absorb’ a radio wave photon with a wavelength of, say, a meter. Yet, these photons slow down the same as others (actually, refractive index varies with wavelength, but it is still > 1.)

Unfortunately, I can’t think of or find in a quick web search a good explanation for why photons slow down in a medium. Perhaps if this topic goes on long enough I’ll dig out my optics text book and see what it has to say.

"Maxwell’s Equations

The velocity of electromagnetic waves has its present value of 299,792.458 km/sec, only in vacuum. When light enters a denser medium, such as glass or water the velocity in the medium drops immediately by a factor of one over the index of refraction (n) of the medium. For practical purposes, the index of refraction is equal to the square root of the dielectric constant of the medium—which is the real part of the dielectric permittivity of the medium. Materials other than vacuum are lossy, causing electromagnetic waves to undergo dispersion as well a change in wavelength in the medium.

For example, the dielectric constant of water at radio wavelengths is about 81, so the velocity of radio waves in water is 299,792.458 / 9 or 33,310.273 km/sec. In the visible light band, n is about 1.3 for water, giving a velocity of 225,407.863 km/sec for visible light rays."

Is the dielectric permittivity the reason? Don’t currently know the physical phenomena associated with this though, i.e. the reason for the change in wavelength vs dispersion effects.

– DD