Why does the speed of light slow down when it passes through something?

[Crafter_Man]

According to Wiki:

In the case of absorption and re-emission, there is a finite amount of time for a material to absorb and re-emit a photon and this lag time will cause an effective “slowing” of the observed photon speed. Between absorptions and re-emissions, however, the photon is traveling at c.

Makes sense to me. But then another Wiki article says this explanation is incorrect:

It is sometimes claimed that light is slowed on its passage through a block of media by being absorbed and re-emitted by the atoms, only traveling at full speed through the vacuum between atoms. This explanation is incorrect and runs into problems if you try to use it to explain the details of refraction beyond the simple slowing of the signal.

Here’s yet a third Wiki article on the subject:

In a classical wave picture, the slowing can be explained by the light inducing electric polarization in the matter, the polarized matter radiating new light, and the new light interfering with the original light wave to form a delayed wave. In a particle picture, the slowing can instead be described as a blending of the photon with quantum excitations of the matter (quasi-particles such as phonons and excitons) to form a polariton; this polariton has a nonzero effective mass, which means that it cannot travel at c.

Now I’m really confused. :smack:

So can someone give me the straight dope on the reason why the speed of light slows down when it passes through something?

[MikeS]

The third one is the most correct, although perhaps a little terse.

The basic idea here is that light waves (all electromagnetic waves, in fact) are coherent excitations of the electric and magnetic fields. In vacuum, they travel through space at c. However, one thing about electric and magnetic fields is that they cause charges to move around; and one thing about moving charges is that they throw off electromagnetic waves of their own.

So when an electromagnetic wave happens upon a material object, made up of charged particles (electrons and protons) that are bound together by complicated force structures of their own, the picture on the microscopic level becomes very complicated. The impinging electromagnetic field causes the charges to move from their equilibrium positions, which causes the charges to radiate electromagnetic fields of their own, whose forces add on to the forces that the impinging wave is causing, which… and so on and so on. It turns out that the only way for these forces and fields to act like a wave inside the material is for them to travel at a speed somewhat less than c; otherwise, the charges in the material end up oscillating out of phase with the initial wave, the resultant radiation interferes with itself and the impinging wave, and the resulting configuration of fields looks nothing like a coherent wave travelling through the medium. Only by having a coherent oscillation of material and external wave can you get something that walks like a wave and quacks like a wave.

All this is on the level of classical electrodynamics, by the way. Things can be explained in terms of quantum mechanics as well (this is where the “polaritons” the Wiki article mentioned come in — they’re the analog of photons in vacuum), but it gets very complicated very quickly and to be honest I don’t entirely understand it myself. Unless some brave & gifted physics teacher happens upon this thread to explain that to you, you’ll probably have to make do with the classical picture; it’s pretty darn accurate anyhow, so don’t worry that you’re somehow being short-changed.