Speed of Light Through a Medium?

Research it then. Learn the physics.

 

Log in or Sign up to hide all adverts.

This certainly isn't graduate level physics. This is basic physics, and should be taught early.

 

Log in or Sign up to hide all adverts.

Alex, does the wave slow down?

Read only,

You’re right. However, this particular kid wanted to know more. When they ask for more, you should give them more. That’s how you should determine their level of understanding. If he’s already thinking about things like this then the information that Alex gave him is completely appropriate.

There’s a good chance that his teacher is dumb. If he’s studying optics then perhaps, he should ask her if rainbows are polarized.

 

Log in or Sign up to hide all adverts.

Seems to me that this thread may have become just a bit sidetracked.

Please Register or Log in to view the hidden image!

The teacher is probably competent for teaching at this grade level and what is being taught IS at the proper grade level.

What is different, in this particular instance is that we have a student who is above average and is interested in following this subject into a deeper level than is taught to the average class. Nothing wrong with that - in fact I think it's excellent! And he's already attempting to learn more than is available in that normal classroom setting. His very presence here - and what he's asking - attests to that.


I also need to note that the expressed motive of "showing up the teacher" is not a good one by any means.

So the proper course of action is to tell him what he's reasonably able to understand and for he and his parents to check on the availability of a more advanced class. Lacking that (the probably best) solution, we should be directing him to some college entrance-level books and let him study at his own pace and even encourage him return here when he hits a point that needs clarification. I'm totally in favor of anyone getting the best education they can.

 

That's great but the point that needs clarification is if it actually slows down in a medium. Two other members claim that the wave slows down.

It does not. The apparent speed slows down because it is absorbed and reemitted by bound electrons in matter which delays the transmission of energy and momentum.

Do you agree?

http://www.steelypips.org/principles/2005_04_17_principlearchive.php#111379031611472347

 

edit

 

Light does not slow down, and the light wave and photon are just two aspects of the same thing.

 

Right, so all wavelengths are actually travelling at speed c between "emission" and "absorption". Thus, the refraction must be caused entirely by the delay between being absorbed and reemitted.

Is there some explanation for the wavelength-dependance of this effect?

 

Yes. Dispersion...

Please Register or Log in to view the hidden image!

http://en.wikipedia.org/wiki/Dispersion_(optics)

This was his original question.

Light is treated as a wave in optics, but between atoms the light is traveling at light speed (c = 3×108 m/s), but after reading about the group and phase velocity. I'm getting a little confused.

Someone who is more qualified needs to answer this.

http://en.wikipedia.org/wiki/Dispersion_(optics)#Group_and_phase_velocity


But here’s a good interactive link for him and you have to admit that it is age appropriate…

Please Register or Log in to view the hidden image!

http://www.colorado.edu/physics/2000/index.pl

 

You too, Alex.

 

I haven't read this whole thread, but I've looked at the last few comments. Hopefully this cursory reading has given me enough information on the problem. If I'm off-base, let me know, and I'll try to clear it up tonight.

The confusion comes because you're not separating the classical and quantum behaviors of the light. You are allowed to think of the light as a wave, or as a particle, but not both at the same time. So, if you want to think of light as a wave (i.e. optics), then you must forget anything you know about atoms. Classically, there is a medium---there are no atoms.

If you want to think of light as a particle, i.e. the quantum picture, then you can think about atoms. The light travels through vacuum, and encounters atoms when it enters the medium. In the space between atoms, of course, the light is in vacuum. In the material, though, the light has a higher chance of hitting an electron, geting absorbed, then being reemitted, which we interpret (classically) as a change in the speed of light.

Did this clear up any confusion?

 

Let me reiterate: one is free to use the classical picture or the quantum picture to describe, but not both. The quantum picture should explain the classical picture.

 

Nope, nope, not clear to me.
Holy crap! I thought I knew this but I don’t…

Please Register or Log in to view the hidden image!

Ben,

Wait a minute, this is basically indicating the same thing. That it is just the apparent speed that slows down because it is absorbed and reemitted by bound electrons in matter which delays the transmission of energy and momentum? One gives the reason why it slows down and seems to correctly state that the light itself isn't slowing down.

That's what Alex said, right?

Are you saying that you can use either one, but just not both simultaneously. That both statement are correct? It slows down is correct, but it doesn't slow down is also correct.

Is there any chance that you would throw in a little bit of the Copenhagen Interpretation for me?

Thanks!

 

Agreed.

Please Register or Log in to view the hidden image!

And allow me to add just another tiny bit to the picture as requested by Trooper in a PM to me.

In any action, such as an energy conversion, there is at least a TINY amount of time required to complete the process - and in this situation there are two conversions taking place: First, the photon strikes an electron and secondly the electron falls back to the ground state and releases an electron. And of course this process is repeated *millions* of times and those times are additive. So th end result is an over-all delay in passing through the medium. And, just as you said, we interpret - classically speaking - that delaying effect as a reduction in speed. And that interpretation is useful and works quite well however you might choose to use it.

As a rough parallel example, consider how we view and use "centrifugal force" which is actually not a force in itself but rather a result of plain old inertia in action.

 

I guess Read-Only meant to say: "First, the photon strikes an electron and secondly the electron falls back to the ground state and releases a Photon". Not "releases an electron". Right? I fully understood the explanation though.

 

That's all well and good, but no one has explained why blue light is slower in a medium than red light.

Is light with a shorter wavelength more likely to become absorbed by an atom? Or does it take longer for an atom to absorb and re-emit shorter wavelengths of light?

Those seem like the only two possibilities, considering that all wavelengths of light travel at c when they are in-between the atoms.

 

Neddy Bate, in what now seems like an inappropriate metaphor to give a 10th grader asking questions, I explained below how I think of the physical (quantum) description of what is going on...

 

That makes sense. Thanks, RJBeery!

 

Good catch, Moran, that mistake was the result of trying to type too fast. Sorry, folks.

 

Here was my teacher's response. People on science forums on the internet are not scientists. They are only people who like to discuss science. I doubt that any of them even have a bachelor’s degree in science.

Light can exhibit both wave and particle characteristics but when discussing the speed of light traveling through any material it is always observed as a wave. When light travels across a boundary between two media it changes direction and it speeds up or slows down. It also changes the wavelength into a wave with a larger or a shorter wavelength.

Thanks for nothing!