Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Phase shift upon external reflection

  1. Dec 8, 2008 #1
    Hello, this question might not be particularly clear, but I'd like a quantum mechanical description of the classical idea of external reflection by a medium. Now, the way I see it, a photon (or numerous photons if necessary) approach the boundary of a medium. Classically, the reflected wave has a phase retardation of 180 degrees. I figure this same effect must be observed quantum mechanically, leading to interference of photons in a way corresponding to the classical picture. My problem is seeing where this effect comes from and why it's not observed for internal reflection. To quote http://www.mellesgriot.com/products/optics/oc_2_1.htm

    Does anyone have any insight on why this is? Or a way for me to grasp it. My maths is sufficiently complicated and my knowledge of quantum mechanics good enough that any answer could get quite technical if desired, but it would be nice to have an answer which argues with basic physical principles and easy to understand language at the same time.
     
  2. jcsd
  3. Jul 5, 2011 #2
    To motivate the phase change in a photon I can use the case of a infinite barrier with a particle and show you that the phase change is true. The whole idea is using the correct boundaries conditions. Now this is not a good answer and also don't answer your second question. (which answer I don't know, but probably someone here in the forum knows)

    The approach that apparently is best for the fundamentals ideas of the reflection of a photon is in terms of Quantum Electrodynamics. There is two important things that I can tell you. First the internal reflection depends of snell's law which don't need to be obeyed by a single photon. The reason is that using the idea of path integrals the photon can take any path without constraints. Second the photon is not reflected as you think. The point here is that the photon is absorbed at the material and latter another photon is emitted. This process when is considered by many photons will in theory give the reflection law and snell's law. Now the calculations are complicated and for that reason I will not try it, but I doubt that there is not a paper that discuss this issue.

    Now the practical approach is the one that you know based in classical dynamics. Probably there is also an approach that can be motivated by some model in solid state physics, but also I ignore it.
     
  4. Jul 5, 2011 #3
    If so, not any interferometer would ever work, not any "Zeno effect" in a Mach-Zender interferometer would ever work, not any Marlan-Scully type experiment would ever work.

    And our telescopes would not work at all, either. And I am sure my telescopes are working, the Newton ones and the Maksutov-Cassegrain too.

    Even the prisms of our binoculars would not work, too...

    You are fully in error, there.
    Simply you have forgot the physical reasons of the Principle of Fermat.
     
  5. Jul 5, 2011 #4
    I think you misunderstood what I mean. From a special relativity point if a photon is reflected then it mean by continuity that in some moment the velocity of the photon should be zero and then the photon is a rest. (that is a problem.) That is the problem by thinking that the photon was just reflected.

    What really happens is that the photon interact with the material and is absorbed and latter is emitted (there is chance that the photon will pass trough, but this chance is related to the transmission and not to the reflection). This is the same concept that we use in quantum mechanics when a photon is absorbed by a atom. Then the state will relax an emit a photon. This doesn't contradict any of the concepts that you mentioned before, then I don't see the problem. This is really what is happening from a quantum mechanics point of view. A good discussion about this that doesn't need a background in QED is the one made by Feynman in their lectures about QED.

    If you don't believe me seek in Google for an answer of how a photon is reflected by a mirror (use QED also in the search to eliminate answer based in macroscopic concepts a refraction index and optics)


    Good Luck
     
  6. Jul 5, 2011 #5
    Are you kidding us ? A photon at rest ?

    Obviously - by the enormity of your illusions - you do not have any experience in astronomical optics, you have never inspected the quality of a mirror, and you do not even suspect how it is done.
    All astronomical optics is undulatory.
    Period.

    The correct laws have already been established, during the 19th century.
    On our Newton reflectors, only our eyepieces benefits from anti-reflection technology, born in the 20th century. Quarter-waves layers, so on.
    But all that remains undulatory.

    The corpuscle you imagine has never existed and will never exist.
    The best a mirror or a lens is, the less it is absorbing.
    Whether one particular photon is transmitted or reflected by a diopter does not depend of any initiative from the diopter, but on the transaction between the initial emitter and the final absorber. Lots of experiments have proved that.
     
    Last edited: Jul 5, 2011
  7. Jul 5, 2011 #6
    Seriously were have you been during the last century?

    Anyway is not my theory. I am just telling you what quantum electrodynamics said. All the experiments that confirm one of the most amazing theories in physics are just illusion, because you think that the mirror in ur telescope is perfect.

    Read my friend, read. There is a century of physics that u r missing.
     
  8. Jul 5, 2011 #7
    Surely you are joking...

    Surely you are joking, Mr "chwie". Just kidding us.
    So you have quite a new optics... Please explain it to astronomers and opticians.
    I am willing to observe the happening. :smile:
    :rofl:
     
  9. Jul 5, 2011 #8
    I don't need to explain it to opticians, they know that what I am saying is true.

    I suspect that you don't know quantum mechanics, then I suppose that a good book is: ( I hope that is the case)

    http://www.amazon.com/QED-Strange-T...4170/ref=sr_1_1?ie=UTF8&qid=1309919804&sr=8-1
    This book can be found in internet to download.

    Also I told you to use Google or wiki. i will post some links of wiki:
    Optics
    http://en.wikipedia.org/wiki/Optics (there you will learn that there is something called quantum optics)
    Quantum optics
    http://en.wikipedia.org/wiki/Quantum_optics (there you will learn the fundamental description of optics is based in QED)
    http://en.wikipedia.org/wiki/Quantum_electrodynamics
    (there you will learn that I was right)

    You mistake is to think that optics is about lens. mirrors and light. That was true probably in the eighteen hundreds. You missed a lot of physics.
     
    Last edited: Jul 5, 2011
  10. Jul 6, 2011 #9
    As far as I can read, the topic is about reflections on mirrors and diopters.

    And there are very few experimental devices where a photon can be stationnary and "at rest" (Ulm experiment, 2007, 9th harmonic of the interferometer).
     
  11. Jul 6, 2011 #10
    Here there is a video that present some of the examples of Feynman in the lectures about qed.



    The complete example of the mirror is not included (adding also absorption, there is only the path integral part), but there is enough concept to understand what I said before. Is about 20 minutes.
     
    Last edited by a moderator: Sep 25, 2014
  12. Jul 9, 2011 #11
    Mirrors and fairy tales...

    Galileo was threatened of been tortured (perterritio verbalis) because he had proved that Aristote and the Book of the Jews were fallible, and wrong.
    I will disclose you a terrible secret : even Richard Feynman was fallible too, and had taught heavy mistakes.
    Will you burn me ?
    I maintain that you do not have any professional experience of interferometers, nor even amateur practice of telescope mirrors. Even lasers would not work at all if you could dictate their properties to conform to your dreams : "First absorption of a photon, then asynchroneous emission of a different photon, just for energy equilibrium". You have described the thermic equilibrium, say of a black body or a real body, and you pretend it is the behaviour of mirrors. You do not know mirrors.
     
    Last edited by a moderator: Sep 25, 2014
  13. Jul 9, 2011 #12
    The story of galileo is not that simple, but that is not the point.

    Mirrors are made of atoms (use an scattering experiment to prove it). Light is composed of photon (use a photomultiplier to check it). Mirrors are not perfect (use a Tem). Atoms absorb an emit photons (use experiments as the fransk hertz experiment to prove it). Then how is that mirrors that are composed of atom don't absorb photons?

    Your problem is not with Feynman is with nature.

    Laser (Light Amplification by Stimulated Emission of Radiation) are also produced by effects as the ones that you are assuming to be false. If particle didn't absorb and emit photons then we will not have lasers.

    I suppose you are astronomer (or amateur in astronomy). Is true that astronomers use high quality mirrors and that as the sources are far away they don't need to use quantum considerations. Now they are not the only one that use optics. In solid state physics optics is used as much as in astronomy. This is the method to observe the crystal and whatever the want to study about the structure. Rotation modes of the atoms, vibration modes, ect. They use optics, but not only the concept of mirrors. Like they study crystal they need to consider in many cases the interaction of an atom with a photon. Then they know a lot of optics and they know that is true that matter absorbs photons and that mirrors also do it. Then if you want to discuss this with an expert in optics then seek for an experimental solid state physicist, they know a lot of optics and know that what I am saying is true, because they use it as a key feature in many of their methods.
     
  14. Jul 9, 2011 #13
    The mirrors are not the defects in the mirrors.

    And what are lasers made of at both end, if not mirrors ? Only good quality and good alignment of the mirrors permit lasing.

    You have not yet perceived the difference between mirrors and defects in the mirrors.

    Yes the defects are absorbers. The bulk of a good mirror is not, and is it used because it is not made of absorbers (or so few). So no phase is lost. The both sides of a solenoid in an Aharonov-Bohm experiment are not absorbers too, they are simply the two branches of a path for each interfering electron.

    In radiocrystallography too, we use the mirror properties of the crystallographic planes - as imperfect are they. We can not avoid absorption, but for that operation, it is just a drawback and a cause of errors.
    Things would be quite different if we practiced an X-fluorescence or used a microprobe.
     
  15. Jul 9, 2011 #14
    What I am saying about mirrors doesn't change the practical behavior of them, just that what is happening is not that simple as you thought. To give you examples that are not related to mirrors. Contact forces as touching and punching are really manifestation of electromagnetic forces. That doesn't change the behavior, just that what is happening is more complex. Also friction is the result of electromagnetic forces between two irregulars surfaces. When you trow a ball against a wall the reason that the ball is reflected is because of electromagnetism interactions (photon and electron interaction). All these behaviors can be explained in a simple picture but in reality what is happening is something more complex. The same is for mirrors. You can explain how a mirrors works using some simple axioms that have been confirmed experimentally. Now what really is happening is more complex. I don't understand your resistance to accept the fact that light is not reflected as you thought, but that in practice the rules that you know are enough.

    This is not a theory, this is a fact. Matter interact with photons, mirrors are made of matter. Now the word mirrors can be used in many sense. Normally refers to something that can create the apparent effect of reflecting of a particle. For example a object that reflect an electron will not absorb the electron, but it will interact the electron by sending an absorbing photons producing the effect of a reflection. In the case of light an common mirrors the photon will be absorbed if it interact with an atom and latter i will be emitted. This effect give the impression that there was a pure reflection, but that is wrong. Not important how well the mirror is made the only thing that need the electron to be absorbed is the presence of atoms. There is not a pure elastic collision between atoms and photons. The best thing that can happen is that the atom absorb and emit of the same frequency as the original one. The other case will be inelastic in the sense that the energy of the emitted photon will be different than the energy of the absorbed photon. This idea explain not only total reflection, but also partial reflection. Things interact by four fundamentals forces and only this four fundamentals forces can change the momentum of a particle, but sometimes is not obvious how the act.
     
  16. Jul 9, 2011 #15
    Now you have explained why you are reasoning so oddly : you think corpuscles.
    Not surprising that the result is such a disaster.
    This way, you simply cannot at all understand mirrors nor diopters, nor Bragg diffractions.
    Your education in physics is to repair thoroughly. A big careening, with lots of faulty pieces to change...
     
  17. Jul 9, 2011 #16
    Ok I tried to explain it as simple as possible. Obviously you are not a graduate student in physics (first year of graduate studies in physics are enough to understand what I am saying). I gave you reference, videos and links in Wikipedia. My advice is to take a course in quantum mechanics, and also if you can in quantum field theory. If you don't believe in these theories, prove them wrong by an experiment, because there are a lot of experiments that prove them to be right. You say that I don't understand mirrors nor diopters, nor Bragg diffraction. Are you joking? I tried to explain you the fundamental interactions that are responsible for all optics and you just didn't understood it or accepted it. I know I am right and that you are missing a century of knowledge in physics.

    Just in case the ideas that I presented don't produce a disaster, they produce optics.

    It was fun, but seriously I feel as I am wasting my time. Nobody can say that I didn't try.

    Good luck!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Phase shift upon external reflection
  1. Scattering phase shift (Replies: 1)

Loading...