Speed of Light Through a Medium?

[Dywyddyr]

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

 

[arfa brane]

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.

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.

 

[Pete]

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.

 

[brucep]

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.

 

[Mike Hawk]

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.

 

[brucep]

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.

 

[brucep]

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.

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

 

[arfa brane]

The frequency should make no difference just like in the photoelectric effect but refraction and dispersion does depend on the frequency.

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

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.

Her explanation is correct in the classical domain.

 

[BenTheMan]

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.

re 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.

Sorry...was that not clear?

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

Thanks!

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

 

[Mike Hawk]

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?

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.

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.

“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.”

made by me

 

[prometheus]

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.

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.

 

[Pete]

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).

 

[Read-Only]

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>

 

[brucep]

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).

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'.

 

[arfa brane]

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?

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.

 

[Trooper]

Sorry...was that not clear? The Copenhagen, why? I don't really like the Copenhagen interpretation, and it's not really relevant here.

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.

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.

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.

Most of the time, two competing theories can’t exist to describe one phenomenon. But in the case of light, one theory is not enough. Instead of throwing out one theory and keeping the other, physicists maintain a wave/particle duality to describe the behavior of light. It is important to understand that this is not an "either/or" situation. Duality means that the characteristics of both waves and particles are present at the same time. The same beam of light will behave as a particle and/or as a wave depending on the experiment. The wave form of light is actually a form of energy that is created by an oscillating charge.

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…

 

[James R]

Mike Hawk:

I have skimmed through this thread. I've seen a lot of correct responses, and some confusing ones. I'd like to reply myself, so excuse me if I repeat some of what other people have said before.

In the classical picture of light, light is a wave. The speed of a light wave is fastest in a vacuum and reduces in any other medium. The speed of a wave in a medium is v = c/n, where c is the speed of light in vacuum and n is the refractive index of the medium. The speed of red light is different from the speed of blue light in most mediums due to the properties of the medium. Another way to say this is that the refractive index, n, of a medium depends on the wavelength of the light passing through the medium.

In the quantum picture of light, light consists of small(ish) packets of energy that move through a medium. Mediums, on the small scale, are mostly empty space, with atoms every now and then. When a photon is moving between atoms, it always moves at c - the speed of light in a vacuum. But when it interacts with an atom in the medium, that interaction takes a little time. In effect you can think of the photon being absorbed then re-emitted by the atom, although that's simplifying the picture a bit. That absorption and re-emission process causes a delay in the total time that it takes a photon to travel through the medium, leading to a lower "effective" speed of light that matches the classical formula v=c/n. The dependence of n on the wavelength has to do with the quantum energy level structure of the particular medium.

But don’t all other waves, like sound waves, travel faster through a medium?

It depends on the medium and the type of wave. Look at something like waves in water, for example, and you find that the water depth affects the wave speed - waves travel slower in shallow water and faster in deep water.

I believe that light does slow down through a medium. There are lots of experiments to prove it. I even found one that showed that they stopped light completely.

I just don’t understand how we know for sure that longer wavelengths travel faster through a medium than shorter wavelengths.

Snell's law of refraction is derived using the assumption that the speed of light changes in a medium. Since we see red light refracting through a prism differently to blue light, in just the way that Snell's law predicts, we know that the assumption behind Snell's law must be correct.

Could you explain explain the Fermat's Principle that light follows the path of least time.

Fermat's principle comes after observing that light travels at different speeds in different mediums. The question is: once you know the speed of light in, say, air and glass, can you predict what path a light ray will take when it goes from air to glass, and vice versa? The answer is: yes. If you assume that the total travel time must be minimised, then you can derive Snell's law of refraction.

Since the speed is constant, the minimum time path is simply the minimum distance path.

Fermat's principle was invented precisely to deal with two (or more) different speeds, for light going from one medium to another.

Then I found this that says that the original statement of Fermat's principle was, "The actual path between two points taken by a beam of light is the one which is traversed in the least time." Snell's law and the law of reflection follow directly from this statement. It may be reformulated slightly in terms of optical path length as "Light, in going between two points, traverses the route having the smallest optical path length." In its original form however, Fermat's principle is somewhat incomplete and even slightly in error. Its modern form is "A light ray, in going between two points, must traverse as optical path length which is stationary with respect to variations of the path." In this formulation, the paths may be maxima, minima, or saddle points.

This is correct. Note, however, that the term "optical path length" is NOT the same as "path length". Shortest time (Fermat) doesn't generally mean shortest distance (length).

I tried to see if there was an experiment that showed that higher energy traveled slower and I did find one that showed that higher energy gamma rays traveled slower than the speed of light. This confuses me even more because the speed is supposed to be constant in a vacuum.

Was that for gamma rays in a medium or in a vacuum? It sounds like maybe you misinterpreted the experiment.

Light moves slower through denser media because more particles get in its way. Each time the light bumps into a particle of the medium, the light gets absorbed which causes the particle to vibrate a little and then the light gets re-emitted. This process causes a time delay in the light's movement so the more particles there are (the more dense the medium), then the more the light will be slowed down.

That's the quantum picture described at the start of this post.

So light never loses any energy or speed and technically it always travels at c. It is always a constant even in a medium, right? So technically light never slows down no matter what. They should tell you this right up front, don’t you think?

No. I don't think so. Looking at what happens on a macroscopic level at first makes much more sense than trying to guess what is happening on an invisible, microscopic level. You can get a prism and refract a ray of light through it, measure angles, and just look at it. But you can't see individual photons or atoms. You need a much deeper understanding to grasp what is happening on the microscopic level.

Okay. Technically light never loses any energy or speed when traveling through a medium it always travels at c. It is always a constant even in a medium. So technically light never slows down no matter what. So light never loses speed or energy when traveling through a medium.

That's the finally correct answer, right?

Summary: in a quantum-mechanical picture, the speed of light between atoms in the medium never changes, but the effective speed of the light as it crosses some macroscopic distance in a medium is reduced compared to the vacuum speed. The amount by which it is reduced is exactly given by the usual classical laws of refraction (Snell, etc.)

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.

Quite a few of us here have at least a Bachelor's degree in Physics. Some of us even have PhDs in physics and teach in universities.

Your problem in this thread (and in general on sciforums) is that you have no way to tell if the person giving you a particular answer has any qualifications or not. Some people here will give you rubbish answers or just get things wrong. Fortunately, there are enough qualified people here that they will usually point out the errors those other people make quite quickly.

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.

That's not quite right. It really depends on what exactly is meant by "observed". What kind of observation are we talking about? Who is making the observation? What exactly are they doing?

Hope this helps.