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When light enters a medium, it slows down. I have heard that this is because it is constantly exiting electrons in the medium and subsequently being re-emitted by those electrons a short time later. Why does it then travel in a straight path? I thought that electrons emit photons in a totally random direction, which would disperse a beam quite quickly.
Do Photons Move Slower in a Solid Medium?Contributed by ZapperZ. Edited and corrected by Gokul43201 and inhaThis question appears often because it has been shown that in a normal, dispersive solid such as glass, the speed of light is slower than it is in vacuum. This FAQ will strictly deal with that scenario only and will not address light transport in anomalous medium, atomic vapor, metals, etc., and will only consider light within the visible range.The process of describing light transport via the quantum mechanical description isn't trivial. The use of photons to explain such process involves the understanding of not just the properties of photons, but also the quantum mechanical properties of the material itself (something one learns in Solid State Physics). So this explanation will attempt to only provide a very general and rough idea of the process.A common explanation that has been provided is that a photon moving through the material still moves at the speed of c, but when it encounters the atom of the material, it is absorbed by the atom via an atomic transition. After a very slight delay, a photon is then re-emitted. This explanation is incorrect and inconsistent with empirical observations. If this is what actually occurs, then the absorption spectrum will be discrete because atoms have only discrete energy states. Yet, in glass for example, we see almost the whole visible spectrum being transmitted with no discrete disruption in the measured speed. In fact, the index of refraction (which reflects the speed of light through that medium) varies continuously, rather than abruptly, with the frequency of light.Secondly, if that assertion is true, then the index of refraction would ONLY depend on the type of atom in the material, and nothing else, since the atom is responsible for the absorption of the photon. Again, if this is true, then we see a problem when we apply this to carbon, let's say. The index of refraction of graphite and diamond are different from each other. Yet, both are made up of carbon atoms. In fact, if we look at graphite alone, the index of refraction is different along different crystal directions. Obviously, materials with identical atoms can have different index of refraction. So it points to the evidence that it may have nothing to do with an "atomic transition".When atoms and molecules form a solid, they start to lose most of their individual identity and form a "collective behavior" with other atoms. It is as the result of this collective behavior that one obtains a metal, insulator, semiconductor, etc. Almost all of the properties of solids that we are familiar with are the results of the collective properties of the solid as a whole, not the properties of the individual atoms. The same applies to how a photon moves through a solid.A solid has a network of ions and electrons fixed in a "lattice". Think of this as a network of balls connected to each other by springs. Because of this, they have what is known as "collective vibrational modes", often called phonons. These are quanta of lattice vibrations, similar to photons being the quanta of EM radiation. It is these vibrational modes that can absorb a photon. So when a photon encounters a solid, and it can interact with an available phonon mode (i.e. something similar to a resonance condition), this photon can be absorbed by the solid and then converted to heat (it is the energy of these vibrations or phonons that we commonly refer to as heat). The solid is then opaque to this particular photon (i.e. at that frequency). Now, unlike the atomic orbitals, the phonon spectrum can be broad and continuous over a large frequency range. That is why all materials have a "bandwidth" of transmission or absorption. The width here depends on how wide the phonon spectrum is.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 accumulate the delay.Moral of the story: the properties of a solid that we are familiar with have more to do with the "collective" behavior of a large number of atoms interacting with each other. In most cases, these do not reflect the properties of the individual, isolated atoms.
There's too much in that quote to deal with at once but there is a serious error when he states that atomic transitions in a solid correspond to discrete frequencies. In the solid state, the energies occur in bands, so the material will interact with a continuous spectrum and not with lines.
Glass does have dispersion and absorption at higher frequencies than light, presumably because of the band structure.Even if the energy re radiated from each atom is omnidirectional, the sum of all re radiated photons will just be a delayed version of the incident energy. The Huygens argument says that the resultant wave will be in the direction of the incIdent wave.
I don't think the situation is the same as scattering in a gas because the scattering centres are smaller and closer together.Having looked at the reference, it seems thet the particle approach is just harder. It seems to be just standing up in a hammock for the sake of it. (lightarrow you may need to search for that reference. I'll be interested if you 'get it')The way that a solid behaves 'as a bulk' seems to be the way to deal with it.
How is it that you can treat isolated atoms / molecules as such broad band scatterers, I wonder, when they have characteristic absorption lines? I can understand that dust particles would be broadband but . . . molecules?
But why should it do that? How can there be any interaction (gaining or losing energy) that doesn't fit the QM model of a molecule? At first sight, you seem to have to ignore the discrete energy levels of the atomic system - and that can't be right.
Electrons in a metal only do what they do because of the band structure due to the vast number of nearby atoms and the Pauli thing.
Where is the equivalent effect for an isolated gas molecule? Does the whole atom / molecule 'vibrate' in space because of a non-zero effective charge? I could imagine that would have a non discrete set of energy levels which could explain things. What does make sense to me is the possibility of Raleigh - type scattering for small conducting objects (particles of metal, perhaps).
If you believe Richard Feynman, because photons have no mass, they can travel at the speed of light. Because time slows down the faster you go, the photons that make up light never age, therefore, theoretically, they can be everywhere in the universe at any one time. Now, according to Feynman, light takes all possible paths to reach a particular destination. When added up as vectors, the sum of all of those paths gives the direction that the light we see has travelled. For simpler purposes, this is 'always' the shortest distance between the light source and where it is seen, and the photons themselves always travel in straight lines in every direction.
This is where the whole particle/wave problem comes into play. I can't fully explain, but interference, diffraction and refraction are all properties occuring due to the wave characteristic of light, while my explanation was referring to the particle model and I can't explain how the particle/wave theories are linked.
Most of us probably accept that "RF" and light are really just manifestations of the same phenomena (at different frequencies), but I don't think we should assume that, just because we use the term "RF" they are electromagnetic phenomena.
We can transmit and receive light and RF by electromagnetic means (antennae), and we can transmit and receive light and RF by non-electromagnetic means
(although electrons are certainly involved). The transmission method and the reception method need not be the same.Certainly, electromagnetic models work well at the lower frequencies
and photonic models work well at the higher frequencies, but just because they do, it does not mean we should infer too much about what is going on in space between the transmitter/source and receiver/receptor.
geezerWhy would you want not to have waves? All observations of the way light and radio travel, point to the wave properties (diffraction, for a start, then the behaviour in guided wave structures). Maxwell relates varying electric and magnetic fields in space is pretty well established and supported by much evidence.To consider that these waves travel through the medium of space is no harder than anything that you may be proposing. If we are having to stretch our imaginations then the less we need to stretch them, the better imho.What "non-electromagnetic means" do you suggest for transmitting and receiving light and radio? I can't think of any.
While I do appreciate that wave theories and particle theories provide excellent prediction models, I am conflicted that there are two. I am probably more interested in trying to understand the nature of space, even though I'm sure I never will.Perhaps Lightarrow, who has lots of knowledge on the subject, can point me to some helpful references?
Knut Overskeid asked the Naked Scientists:When light enters a medium, it slows down. I have heard that this is because it is constantly exiting electrons in the medium and subsequently being re-emitted by those electrons a short time later. Why does it then travel in a straight path? I thought that electrons emit photons in a totally random direction, which would disperse a beam quite quickly.
Lightarrow:Apologies, as this post is not directly related to this thread, but would you happen to know of a good reference for the famous double-slit experiment? I have a lot of "how-do-we-know" type questions about the experiment. I'm sure they have been asked by countless people over the years, and they probably should not be asked in this forum.
Lightarrow:The Fynman lectures are excellent. Thanks!I have found references to fairly recent double-slit experiments demonstrating that single electrons interfere with themselves. Can you point me at any similar experiments conducted with photons rather than electrons?
One of Fynman's statements is troubling me. While explaining the interference of waves as they reach the screen, he said that the peak amplitudes of the lower amplitude constructive interference distributions were a consequence of interference between the crest of one wave and the crest of the subsequent wave. Of course, this makes sense when there is a continuous supply of wave crests, but I would think it is unlikely in the case of a single photon, speedy as they are.
We cannot think to a photon as a localized corpuscle and even if we could, we cannot (at the moment) say that is "made of" em waves or anything else.
Knut Overskeid asked the Naked Scientists: HiWhen light enters a medium, it slows down. I have heard that this is because it is constantly exiting electrons in the medium and subsequently being re-emitted by those electrons a short time later. Why does it then travel in a straight path? I thought that electrons emit photons in a totally random direction, which would disperse a beam quite quickly. Thanks!Knut OverskeidWhat do you think?
When water waves pass over a piece of glass at an angle in a ripple tank, they change direction. If we find the incidence and refracted angle, can we find the refractive index of the glass?