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Light as a wave, which wave is the correct wave?

  1. Jul 22, 2013 #1
    Light as a wave, which wave is the "correct" wave?

    Okay so when describing light as a wave, I get two different images depending on what Im looking at.
    So when Im looking Young's double slit experiment I see light as "wave fronts" like in this picture:


    But in the photoelectric effect light is show as this type of wave:


    So when talking about light as a wave, which wave is the "correct" wave? I ask this because the photoelectric effect shows why light isn't a wave but if you use the "wave fronts" light then there doesn't seem to be a problem.

    My second question. When light is shone through a single slit, you get a maxima in the middle and some other maxima like this:

    How can this be explained using phasor arrows where the photons dont interfere with themselves?
  2. jcsd
  3. Jul 22, 2013 #2


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    Actually, photons DO interfere with themselves. The common pattern that you see is all due to single-photon interference.

    If you want to see how QM tackles all of those interference effects, you can look up the Marcella paper. All the so-called wave properties are derivable without having to switch gears between our classical idea of wave or particle.


  4. Jul 22, 2013 #3
    When you say photons interfere with themselves. Does that mean that the explanation using phasor arrows is like the explanation using waves in that it's a "dumbed down" version of what actually happens?
  5. Jul 23, 2013 #4
    Both are correct. These 2 pictures are not 2 different things. They are just simplified drawings of the same thing (2 ways of drawing the same wave). The electromagnetic waves are 3 dimensional: in evey point in 3D space there is a different vector. And the whole 3D structure is changing in time. This is very difficult to draw on paper, so we are using these simplified pictures.

    For example the wavefronts may be symbolizing the locations of the maxima of the wiggly waves.
  6. Jul 24, 2013 #5
    The author of the cited article talks about "Particle scattering from slits". What does a photon/electron passing a slit undergo, interaction or diffraction?

    "Scattering" is an interaction. But is "diffraction" also an interaction? I think it is not. A wave diffracts with itself. Can we say that a photon interacts with itself?

    I think a phonon passing a slit only undergoes diffraction; no interaction (like scattering).
  7. Jul 24, 2013 #6


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    This is a type of confusion from the socalled "wave-particle duality idea", I've not heard of so far. It shows that bad concepts lead to a lot of confusion.

    Diffraction of a classical electromagnetic wave is due to the interaction of the electromagnetic fields with objects built out of electrical charges around it. You can also see it as scattering of the electromagnetic on these objects due to interactions with the charges: When the wave hits the object it exerts forces on the charges, which due to the so caused acceleration themselves start to radiate electromagnetic wave fields that interfere amongh themselves and with the incoming wave field. This leads to the characteristic interference pattern.

    Classical particles are scattered due to forces exerted on them, but they do not show interference effects.

    Now in a short period from 1900-1925 one realized that the so far very successful concepts of classical physics, namely mechanics and classical electrodynamics, are not applicable in the realm of the very small consituents of matter. In this period one tried to build a new theory to deal with phenomena that clearly contradict the predictions of classical models. Among them was the Planck Law for black-body radiation (which started the whole "quantum business" in 1900), the specific heat of solid bodies at very low temperatures, etc. etc. The idea the physicists (like Einstein, Planck, Bohr, etc.) came up with was that in the quantum regime entities like the electromagnetic field or an electrons sometimes behave in a particle-like and sometimes wave-like. They always knew right from the beginning that this is not a consistent picture and that a more consistent theory has to be found, before one could be satisfied with the description of nature in terms of its most elementary constituents (which were not even known in those days).

    The solution of this problem has been the development of modern quantum theory in 1925/26 by Heisenberg, Born, Jordan; Dirac; Schrödinger. According to this most successful theory it doesn't make sense to think about nature in terms of classical particles and classical fields. The most comprehensive model of matter is the Standard Model of Elementary Particle Physics. It's the best confirmed theory ever. It's so successful that it is even a bit ennoying for the particle physicists nowadays since we know that it cannot be the complete picture, and without some clear contradictions from experiment, it's hard to find the right direction to find an even better theory. I've just been at a high-energy particle conference in Stockholm. There the experimentalists presented even more evidence for the correctness of the Standard Model, which of course is also a great achievement of human thinking about nature in any case.

    According to the Standard Model there is no wave-particle duality or other confusing concepts necessary to understand the behavior of matter on the very fundamental level of forces and matter constituents. The only thing that discribes matter are quantum fields. Admittedly that's a very abstract mathematical formalism, but Nature seems to like to behave according to this math rather than following the classical concepts we built from our everyday experimience with macroscopic objects and with classically behaving electromagnetic fields (light). It's, indeed, a very demanding task to explain from the fundamental quantum-field theoretical behavior, why these classical concepts work so well for our macroscopic world. I think it's fair to say that this is not fully understood today, although a lot of progress has been made in the recent decades in quantum many-body theory, the understanding of classical transport theory, hydrodynamics, etc. from the underlying quantum many-body equations (Kadanoff-Baym equations), decoherence through the disturbance with the environment etc.
  8. Jul 28, 2013 #7
    I understand that it's way more complicated than what Im making it out to be. My question comes down to the fact that in my physics lesson I was told that the wave nature contradicted the photoelectric effect. I want to know why it contradicted the photoelectric effect because the two wave photos I showed in my original post are different.

    I was told wave nature contradicts because it's meant to be a continuous wave that hits the electron and not discrete packets of energy like this picture:


    But this picture comes up when I'm learning about diffraction:


    This second picture doesn't hit the electron continuously. Im not asking what light actually is, just what is meant when light is described as a wave. Im asking which one of the above I should think of because I was told the wave nature contradicts the photoelectric effect. But only picture 1 contradicts the photoelectric effect and the second picture does not because one wave at a time hits the electron. Any answers?
  9. Jul 28, 2013 #8


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    The pictures are meaningless here and are just confusing you. The second one in your original post just shows some wavy lines with colors next to them. That's just to show you that different colors come from light having different wavelengths. That's it.

    Classically, waves transfer energy continuously, not in packets, aka quanta. But the photoelectric effect shows that this is NOT the case. In fact, EM waves DO transfer energy in quanta. That's the contradiction right there.

    This picture is meaningless. Ignore it.

    This is just showing you the interference pattern of the wave.

    What is meant is that in many ways light behaves AS IF it were a classical wave. It consists of oscillating electric and magnetic field vectors which have a frequency and wavelength and can be measured. For the most part it follows all the same rules that something like a water wave does, and it interferes and diffracts just as a wave should. But, seeing as how light is not fully described by classical wave laws, it is not a classical wave. Hence why we had to develop Quantum Mechanics to describe it.

    There is no "one wave at a time". The waves in the picture only APPEAR to be separate waves. They are all merely different parts of the same wave that have identical phases.
  10. Jul 29, 2013 #9
    "There is no "one wave at a time". The waves in the picture only APPEAR to be separate waves. They are all merely different parts of the same wave that have identical phases."

    That answer will help me sleep at night. Thank you lol
  11. Jul 29, 2013 #10
    One of the earliest descriptors of a photon was "wave packet". Indeed, the word "wave" is very generic, and it can relate to very different models - consequently different people can mean different things when they use that word.

    Compare: https://en.wikipedia.org/wiki/Photons#Nomenclature

    And one more ref, looks interesting but I have not yet read it:
    http://ajp.aapt.org/resource/1/ajpias/v81/i3/p211_s1 [Broken]
    Last edited by a moderator: May 6, 2017
  12. Jul 30, 2013 #11
    The anti-correlation experiments of photons going through a beamsplitter are fascinating.
    Another paper that discusses this effect can be found online (sorry I may not show the link), with a summary here: http://ajp.aapt.org/resource/1/ajpias/v72/i9/p1210_s1 [Broken]

    And what surprised me is the complete disagreement about interpretation (they cite different anti-correlation experiments but those look essentially the same to me, as they both discuss detection of single photons passing through a beamsplitter):

    "An anti-bunched field can be interpreted
    as one in which the photons do not clump together,
    and hence tend to arrive one at a time. When these individual
    photons strike the beamsplitter, they are either transmitted or
    reflected (but not both), leading to anticorrelations in the
    photocounts at the detectors."
    - Thorn 2004, p.1213, http://ajp.aapt.org/resource/1/ajpias/v72/i9/p1210_s1 [Broken]

    "a single photon passed through a 50-50 beam-splitting mirror
    (the “source”), with reflected and transmitted beams (the “outputs”)
    going, respectively, to “Alice” and “Bob.” They could be any distance
    apart and were equipped with beam splitters with phase-sensitive
    photon detectors.
    [..] the entanglement was between two quantized field
    modes, with one of the modes happening to be in its vacuum
    state. Like all fields, each mode fills space, making nonlocality
    between modes more intuitive than nonlocality between
    particles: If a space-filling mode were to instantly change
    states, the process would obviously be nonlocal."
    - Hobson 2013, p.220 of http://ajp.aapt.org/resource/1/ajpias/v81/i3/p211_s1 [Broken]
    Last edited by a moderator: May 6, 2017
  13. Jul 30, 2013 #12
    it is the form that energy takes as it leaves (is emitted) from the atom. I am told by a nuclear physicist that I must use math to explain this part...but I don't require math for the basic concept...here it is.
    Imagine being the atom and blowing a bubble that does not come out of a loop with soap on it...yet instead energy spheres that expand and forms around your whole body, the atom, and these expanding spheres are moving from the atom at the speed of light...and immediately, another forms and does the same thing. This is the requirement for conservation of energy and the way photons form...and as you will determine if you think about it, the distance of these discrete spheres, from the inside surface of the first, to the outside surface of the second determines wavelength and frequency. This still allows the quantum discrete units, but it takes two at a minimum to tango. So orange light might have a gap from sphere one, to the sphere nested inside it of 620 nanometers (nm) and this then solves the dual slit experimental question of how interference occurs if only one photon is the output during the experiment...because one photon is actually two expanding spheres of energy that have a frequency in the visible light spectrum if the gap between them is within that range.... it makes sense with no math, and I am now working on proving it using Planck's constant with math....
  14. Jul 31, 2013 #13


    Staff: Mentor

    A number of posters have clarified whats really happening eg Vanhees71 excellent post.

    I will limit my comment to one of a general nature. QM is a highly mathematical and subtle subject. Even books meant to teach it at a serious level to undergraduates such as Griffiths well respected and widely used text are not entirely satisfactory. I have that text and Griffith even admits he skirts the difficult issues of interpretation and foundations in favor to learning how to solve problems.

    To really understand the modern view you need to consult a graduate level text. My favorite is Ballentine - QM - A Modern Development:

    Here you will find a correct axiomatic development from just two axioms - yes only two. Stuff like the Schrodingers Equation etc are given its true basis - symmetry - and derived - not assumed. He also examines carefully the issues of interpretation supporting the so called ensemble interpretation which is a minimalist interpretation. It may not be the interpretation you eventually settle on but it does provide an excellent introduction to the issues involved.

    After that I would recommend Schlosshauer's excellent book on decoherence:

    It is not a book about foundational issues per-se. It however gives the detail of a major advance in modern times that has had a big impact on foundations and is well worth coming to grips with.

    Last edited by a moderator: May 6, 2017
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