# Why does light accelerates after being slowed down

ludi_srbin
I read about Cherenkov's effect and how light slows down if it goes through, let's say, water. My question is how it can speed up again after it exits the water?

εllipse
Light doesn't really "slow down" in water. The atoms just interfere with its travel. I think technically the photons are obsorbed and re-emitted, but you can think of it as photons having to bounce around the atoms in order to get anywhere, rather than being able to travel straight on through at full capacity.

So like if you threw a ping pong ball at a bunch of obstacles, and it had to bounce all around to get through, it could (in those perfect [frictionless] physics worlds) get through without slowing down, although it would take longer to get through (due to all the bouncing) than it would if the obstacles weren't in the way.

Last edited:
ludi_srbin
So it actually stays the same, only travels in different directions, due to a higher density of the surrounding?

εllipse
I'm not sure of the technicalities. There will be others here who know much more than me in the morning. Until then, I can tell you that when light does travel it always travels at c. Whether the lack in speed of light passing through water is due to the light "bouncing" around, or being absorbed and emitted, I am unsure of. (I really just mentioned the "bouncing ball" as an easy to visualize example, not to be taken as what really happens to photons.) However, when photons are traveling between two atoms in water, they will always travel at c. There seems to be some confusion about what exactly happens to photons when they hit atoms (and are just passing through a transparent medium), but here's a link for more information:

Start reading at zoobyshoe's comment (#4). That's when they start talking about what happens to light passing through a medium.

But to clarify, light will travel more slowly through water if you just send light through it and time how long it takes to reach a mile, but it's not that the photons are actually moving slower. It's just that they get interrupted along their flight by the atoms in the water.

Last edited:
Staff Emeritus
I think technically the photons are obsorbed and re-emitted,

This is correct.

εllipse said:
I'm not sure of the technicalities.

Absorption and re-emission is a good way of to explaining it. Individual photons always travel at 'c', but they occasionally gets absorbed, held for a while, then re-emitted.

What's hard to explain is why the light is re-emitted in exactly the same direction in transparent materials. A really good explanation of this gets into some difficult quantum mechanics. But the idea that this is what happens isn't particularly hard to grasp, IMO.

Umm, photons simply bounce off molecules. That's why you actually see light in the first place without actually looking directly at the source. If you took a flashlight and turned it on in space, you wouldn't see the light beam unless your pointed it right at your eye.

εllipse
Myriad, we're talking about in the case of light passing through a tranparent medium, such as water, in which case the photons are, as pervect verified, absorbed and re-emitted in the same direction as they were going. As to whether light "simply bounces" when it hits opaque materials, someone else will have to answer that, but in the example you gave, it wouldn't even matter whether the light bounces or is absorbed and re-emitted at an angle.

Last edited:
Staff Emeritus
Gold Member
Umm, photons simply bounce off molecules. That's why you actually see light in the first place without actually looking directly at the source. If you took a flashlight and turned it on in space, you wouldn't see the light beam unless your pointed it right at your eye.
A photon never bounces! It is adsorbed by the atomic structure then reemitted a short time later. The direction taken by the reemitted photon will be determined by QM and material properties.

ludi_srbin
Is that absorbtion responsible for the slow down, and how does it speed up again?

Gold Member
When light passes through a media [like glass], electrons attempt to absorb every photon within arms reach [electrons are notoriously promiscuous]. Whether they succeed depends on the frequency of the light and the quantum states available to the electrons. Electrons may only retain a photon if it has just the right amount of energy to promote it to a higher permissible quantum state. If not, the electron must release it back to the wild. But this does not happen instantaneously, which is what slows the photon. The photon otherwise travels at the same old boring speed of light at all times. A material is transparent when electrons are forbidden to retain any photons of visible light [footnote: due to quantum technicalities, some photons are inevitably captured - no such thing as a 'perfectly' transparent material!], and forbidden to alter their path [a separate, more technical issue]. Some materials are not required to allow the photon to retain its original trajectory. These materials are called translucent. And last, but not least, the closer an electron is to achieving a permitted quantum state, the longer it is allowed to abuse a photon before letting it loose. This is why different colors of light travel at different speeds through the same media.

This is all well and good, except it does not answer the apparent question regarding Cherenkov radiation. Cherenkov radiation is emitted whenever charged particles pass through matter at a velocity exceeding the permitted velocity of light in that media. The charged particles polarize molecules, which then return to their ground state by emitting radiation. You probably think this sounds inconsistent with the preceeding explanation. Just remember, if it sounds too weird to be true, it's quantum physics. The explanation lies in the fact that Cherenkov radiation is only emitted when mass possessing particles [like cosmic rays] pass through a transparent medium.

Staff Emeritus
ludi_srbin said:
Is that absorbtion responsible for the slow down, and how does it speed up again?

It seems as if I've tried to explain this a gazillion times already... :)

OK, I will try THIS approach. If it doesn't work, I quit! :) Using the simplest model out of Kittel, here goes...

Consider a solid as being a chaiin of positive ions and negative electron bonds. It looks naively like this:

.....+.....-.....+.....-.....+.....-.....+.....-.....

When a photon approaches, if it's electric field polarization is alligned with that chain, you then can get something like this:

.......+..-.......+..-.......+..-.......+..-.......

and this

..+.......-..+.......-..+.......-..+.......-..+.......-

IF this oscillation happens to be one of the phonon or vibrational modes that the solid can sustain, the photon is absorbed typically as heat (in some cases, it can be re-emitted also). However, if the solid does not have this vibrational mode, or if the solid isn't a well-defined crystal (such as glass) in which most phonon modes within the visible range don't exist, then this vibration cannot be sustained and the photon is re-emitted.

Why the delay? It is because the + ions are heavy and the - electrons are not massless. There is inertia, and it takes time for the charges to react to the oscillating E-field of the incoming photons.

I'll give it a month before this question pops up again somewhere else. :)

Zz.

Gold Member
Great explanation Chronos. That really clarifies things - for me at least.

Mentor
Sorry if I'm oversimplifying after that, but this may require a simpler answer to get started...
ludi_srbin said:
Is that absorbtion responsible for the slow down, and how does it speed up again?
The absorption is responsible for the appearance of the slow down and it appears to speed up again as soon as it leaves the substance. But a physical photon (oversimplification) only ever travels at C.

Consider a 1 mile road with a two traffic lights in it. If your car only travels at 60mph (accelerates instantly), but stops for 30 seconds at each traffic light, your average speed is 30mph even though you only ever travel at 60.

Last edited: