Speed of Light Through a Medium?

Then your teacher is wrong.
Simple, really.
We have a number of actual, practicing, working, published scientists here on Sci.

 

Log in or Sign up to hide all adverts.

Ask your teacher what happens when single photons are transmitted through any medium. Are they observed as waves, and when they cross a boundary does the wave-velocity change?

I can recommend Richard Feynman's book QED, which I think will help you with your understanding quite a bit, and it's aimed at the general public. I imagine a local library will have a copy.

 

Log in or Sign up to hide all adverts.

But none were talking with Mike yesterday.

Mike, I think you only wanted to hear what you already agreed with. You chose to ignore advice that agreed with your teacher, just because you wanted her to be wrong.

 

Log in or Sign up to hide all adverts.

Mike

Your teacher seems to be a bit of a know-it-all. PHD's post here and the best answer came from PHD Ben The Man. Unfortunately it was probably after you made the mistake of calling out your teacher. The quantum interpretation will recover the classical physics in the classical domain of applicability. That's how it works. Good luck and you might want to reconsider going to a gunfight without any bullets.

 

Brucep - BenTheMan has a PhD in physics? Perfect. You just provided me with a few a bullets. Thanks. I’ll show her your remarks on Monday but I have no idea what this means. “The quantum interpretation will recover the classical physics in the classical domain of applicability.” She’s not just a know it all she is really mean. Everyone hates her.

arfa brane - Cool! I will ask her what happens when single photons are transmitted through any medium. Are they observed as waves, and when they cross a boundary does the wave-velocity change?

BenTheMan- If it's viewed as waves the energy contained in one of those waves should depend only on its amplitude and not on the intensity of the light. The frequency should make no difference just like in the photoelectric effect but refraction and dispersion does depend on the frequency.

So if it depends on the frequency isn’t it more reasonable and correct to view it as a particle when thinking about the speed?

Is this just a case where the concept of a wave is useful to understand it but the quantum picture is more accurate?

Is her explanation wrong or not? Yes or no please.

 

Mike

Domain of applicability just means the domain where a theoretical model applies. Classical physics is physics that isn't quantum in nature. IE mechanics, electromagnetism, optics, special relativity, general relativity.... My guess is you're studying optics? So I'll just say this. The quantum explanation is the empirically correct explanation. The classical theory of optics is still very useful. Snell wrote his law in the mid 1600's a time when everybody thought the speed of light was infinite but could be slower passing through a medium. Experiment seemed to prove that was the case resulting in the development of the refractive index. At that time they didn't even know the speed of light was finite until it was discovered that the constant in Maxwells equations [mid to late 1800's] was actually the speed of light. So light doesn't slow down in a medium it's just described that way if you evaluate the phenomena using the classical theory of optics. No problem, it's all good as they say. The part about quantum mechanics recovering optics at a classical limit just means if you compare the quantum experimental results with the the classical experimental results they essentially have a 1/1 correspondence just interpreted in a different way. Think about this domain thing before you confront your teacher.

 

If you go to Prof Edwin Taylors Home site you can download his QED interactive workbook and simulations. Really cool stuffl

 

You have this wrong. The photoelectric effect does depend on the frequency of the incident radiation.

Her explanation is correct in the classical domain.

 

Sorry...was that not clear?

Why? I don't really like the Copenhagen interpretation, and it's not really relevant here.

 

arfa brane- Yes. I think it is correct. I said “if it is viewed as a wave then the frequency should make no difference.” Her answer is correct and you are a 100% positive? Do you have a degree?

made by brucep


What brucep is saying sounds correct. She is saying that when discussing the speed of light traveling through any material it is always observed as a wave. That doesn't sound correct.

made by me

 

I know for a fact that lot's of people on here are scientists because I've met them, and indeed, I have a masters in theoretical physics and I'm working on my PhD which I'll hopefully be finishing this year. You can tell that to you teacher, and also ask what their degree is in.

 

The problem is that Mike didn't ask yesterday who knew what they were talking about, and chose the wrong stuff to give to his teacher, for the wrong reasons (i.e. he wanted her to be wrong).

 

Agreed, which is why I've given up on this thread. I finally realized, much as you just said, that his only motive here is to slam a teacher that he dislikes. Very childish behavior - and nothing to do with actually learning physics. <sad story, this>

 

He's a youngster who might learn something through this discourse [probably the hard way]. So what set of answers do you think he would get if he asked everyone whether they knew what they were talking about ? I's say it would be the set of 'I know what I'm talking about'.

 

You don't need to have a degree to understand the photoelectric effect. It was "discovered" by Einstein who showed that light (radiation) appeared to behave like particles, which determination contradicts the wavelike behaviour of light as in Young's double-slit experiment. Anyone can find out about both these people and what they did in the experiments, and so today everyone who knows about them also knows that light behaves like waves, or like particles, depending on the experiment you do.

Nobody (and I mean nobody) can explain this dichotomy without recourse to either a classical or a quantum model, but it can't be classical and quantum simultaneously as others have pointed out.

 

I’m sorry. I wasn’t trying to be a smartass, this time. Since, you used Bohr’s complementarity principle, that matter exhibits a wave-particle duality. An experiment can show the particle-like properties of matter, or wave-like properties, but not both at the same time. It is closely identified with the Copenhagen interpretation. So, that’s why I asked.

I like Bruce and he’s right. I know of another member, who feels that debating and challenging enhances his ability to learn. The kid has already pointed out that his teacher is annoyed by questions. <sad story, this>. Curiosity should be encouraged, not stifled. Arfa brane provided a good explanation for the wave particle duality that’s along this line.

Quantum mechanics is difficult and can be a barrier for many students completing an education in physics. The intelligent students will probably understand the mathematics but may still have trouble grasping the physical principles. The physical meanings start to become incomprehensible. I think it’s supposed to be thought of as an extension of classical mechanics.

So, don’t ever let anyone make you feel dumb. Sometimes a little history can help.

Since light seemed to travel only in straight lines, people used to think of light as streams of particles. However, it doesn’t always travel in a straight line. If the medium changes then the straight line rule no longer applies, refraction is a good example. We know that Newton was a big fan of the particle theory but it had some issues. Particles would collide and rebound. It also didn’t offer an explanation for color. The particles would have to be different, but no one knew how. Newton could explain refraction with his theory, but the particle speed had to be increased, when passing from a low density to a higher one.

Huygens’s wave theory could explain all of this, but Newton was heavily admired. It wasn’t until 1801 that Young had revived the wave theory with the Young-Helmholtz theory of color, which was the famous double slit experiment. This severely hampered Newton’s particle theory and other evidence started turning up in favor of the wave theory, i.e. Rayleigh scattering, the color of the sky. Huygens’s idea finally won approval concerning the velocity of light in various mediums. One experiment that someone pointed out earlier was Foucault’s. It supported Huygens’s theory. At this point, the wave theory was finally accepted. Thinkng of light as a wave, lead to many other exciting ideas, i.e. the Doppler Effect. However, the wave theory still had problems and couldn’t explain everything.

You couldn't just say that light was only a wave because there are different types of waves. Water waves are transverse, but can also be a combination of both, and sound waves are longitudinal waves. Everyone thought that light waves were longitudinal, but neither the wave or particle theory could explain double refraction, unless it was considered only as a transverse wave. However, if light was a wave, what was waving?

Transverse waves can only be conducted through solids. Now, that light wave considered a transverse wave, we needed a semisolid substance, not just a fluid, because a fluid can’t hold a sheer force. That’s why the idea for ether was born. I think it was Thompson who tried to build a mechanical model with rotating fluid because vortex rings can be very stable and offer more resistance. Maxwell liked this idea and begins to add to this model, which lead him to understand how light is connected to electromagnetism. He is able to calculate what the speed for a transverse wave would be in this model and is within 1% of c.

Analogies are needed for understanding, but they never deliver perfect insight, that's why it is said that Maxwell's physical descriptions are the equations themselves. It was difficult for even the most well known intellectuals to grasp. He used mostly Cartesian notations, because he did not like vector notations, and France and Germany were using laplacian notations, so it was difficult to translate. Hermann Von Holtz agreed with his results but even he could not grasp the actual physical conditions of this statement. I think Maxwell indicated that this field was primary and charges and currents secondary, the charges and currents were not physical entities themselves but a consequence of this field. Many resisted the idea, and did not want to give up the concept of ether, just like with Newton’s particle theory.

Even Einstein's General Relativity did not contribute to ether, and a lot of people resisted the theory of relativity, and some never accepted it. There were no concrete payoffs to allow you to understand practical mechanics. I think Michelson and Lorentz never even came around to accepting GR. The simplicity, unity, elegance, is what came into play for those who did.

Like I said earlier, the theory of relativity didn’t state that ether didn’t exist, but it did remove the need for it to transmit the force of gravity. However, it was still needed to explain how light, as a transverse wave was transmitted across a vacuum, but Einstein’s special relativity killed it.

P.S. I’m sure someone will follow and make a few corrections. That’s how it works here, but you can tell your teacher that I said she’s stupid for trying to squash your curiosity…

Please Register or Log in to view the hidden image!