# Speed of Light

1. Jul 13, 2010

### RYeag

Light travels through a medium and its velocity is (assuming that the medium has an index refraction greater than 1) consequently slowed. Once the light passes through the medium, though, the velocity at which it travels accelerates back to the rate it had prior to entering the medium.

How did the light gain sufficient energy to accelerate back to its prior velocity?

I was asked this by a family member and am stumped. Any clues?

Thanks!

2. Jul 13, 2010

### ZapperZ

Staff Emeritus
Please start by reading one of the entries in the FAQ thread in the General Physics forum. The photons do not "slow down" while going through the medium.

Zz.

3. Jul 14, 2010

### the_house

The original post never said anything about photons (which aren't really that useful for understanding the bulk behavior of a coherent wave of light that consists of extremely large numbers of photons).

It is true that the speed of a light wave will change when it encounters (or exits) a material. The answer to your energy question is that the speed of the wave is not what determines its energy. It doesn't take any energy to "accelerate" the wave as it exits the material.

4. Jul 14, 2010

### ZapperZ

Staff Emeritus
That isn't quite right either.

The "speed of light" that we typically measure is the group velocity. In fact, in a typical material, that group velocity is the one that is "slowed down".

The photon explanation underlies the observation - it explains what is going on. Since this question was posted in the "Quantum Physics" forum and not in the Classical Physics forum, I made the assumption that the OP was interested in such a description.

Zz.

5. Jul 14, 2010

### the_house

I'm unclear about what was incorrect about what I said. Even if you thought I was talking about phase velocity instead of group velocity, isn't it still different from c in a medium? It seems to be pretty common nomenclature to say that light slows down in a medium.

Also, although it's true that the fundamental description of light propagation is one of a quantized photon field (i.e., QED), I don't think the picture of on-shell photons traveling around at c gives much intuition about the behavior of coherent electromagnetic fields in a medium. As you explain, though, it may be true that this is the kind of description the OP was looking for.

6. Jul 14, 2010

### RYeag

Zz,
"On the other hand, if a photon has an energy beyond the phonon spectrum, then while it can still cause a disturbance of the lattice ions, the solid cannot sustain this vibration, because the phonon mode isn't available. This is similar to trying to oscillate something at a different frequency than the resonance frequency. So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This, naively, is the origin of the apparent slowdown of the light speed in the material. The emitted photon may encounter other lattice ions as it makes its way through the material and this accumulates the delay."

So the aggregate "delays" quantify an overall delay of the photons, but their velocity itself is never mitigated. Did I read that right?

7. Jul 14, 2010

### ZapperZ

Staff Emeritus
Yes, but keep in mind, as stated, that this is a very naive view of what is going on. A lot of important details under the cover have been neglected in that explanation.

Zz.

8. Jul 14, 2010

### RYeag

Thanks for the info. Any recommended reading material? I'd like to learn more...the less naive and more correct view.

9. Jul 16, 2010

### loreak

As photons travel through a material they are bouncing back and forth and being absorbed and re-emitted, so the overall "speed of light" is slower than absolute c because their path is not a straight line through the material.