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The wave nature of light

  1. Apr 5, 2014 #1
    In classical mechanics, light is modeled as oscillating electric and magnetic fields, where one continuously induces the other, giving it the speed of light.

    Then in the 20th century, we discover special relativity which shows that magnetic fields are just electric fields created by length contraction. We also show that every fundamental particle has a de Broglie wavelength that behaves similar to that of a photon. QED shows that the electromagnetic field, as well as the others, is just the exchange of virtual particles. Photons in the case of the EM force.

    So why are we still taught in physics 101 this classical view of the photon when really it's just another (massless) particle in the standard model? I mean any other particle, like a quark, isn't described as an oscillating field of some kind, it's modeled as a particle (in the QM sense).

    So I guess my question is before the 20th, they believed light was a wave. Can we now explain that wavelike behavior as the de Broglie wavelength of the photon rather than the classical explanation? If so, why is the classical explanation still taught?

  2. jcsd
  3. Apr 5, 2014 #2


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    The classical explanation is not wrong, it's just not the whole story. We teach it in Physics 101 because you have to understand it before you can move on to the rest of the story.

    And the rest of the story is much more subtle (and mathematically complex) than "light is a stream of photons"; in particular we cannot explain the wavelike behavior of light in terms of the de Broglie wavelength of photons.

    (If I were in charge of the world it would be a criminal offense, akin to selling alcohol or tobacco to minors, to reveal the existence of photons to anyone who was not enrolled in a serious college-level QM course or equivalent. Before then, it just creates confusion and misconceptions).
  4. Apr 6, 2014 #3


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    In fact, all of the particles ARE described as quantizations of fields in the standard model. That's why they call it quantum FIELD theory. The electromagnetic field and the gravitational field are the first two one usually encounters. But all the elementary particles are described by field theories (e.g. spin 1/2 fermions are quantizations of the Dirac field). If one did not learn the field description of E&M, one would have no idea how to quantize it!
  5. Apr 6, 2014 #4


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    My PhD is in laser physics; I have had half a dozen courses in electromagnetic field theory and optics, covering many topics. Some of the material required a quantum treatment: four wave mixing, fundamentals of lasers, x-ray scattering, etc. But most of optics can be understood in terms of waves or even rays.

    From a historical and didactic point of view we have at least four workable theories of optics:

    1. Ray theory: works for simple optical problems such as lenses and mirrors. Fermat's Principle of Least Time is based upon ray optics; from it can be derived the eikonal equation. Very useful and powerful stuff.

    2. Wave theory: Huyghen's wavelet construction shows how a wavefront propagates; the normal to the wavefront is the ray in the direction of propagation. Thus wave theory (circa 1675) subsumes ray theory, plus includes interference and diffraction effects. Young's double slit experiment (circa 1800) showed that waves are better model - required for some details. The modern theory of microscopic imaging, Abbe, 1870, is modelled on diffraction.

    3. Physical wave theory: Maxwell's equations explain polarization of light - though of course polarization was known, if not completely understood, for a long time previous. From Maxwell's electrodynamics comes an understanding of how light is generated, transmitted, scattered, and absorbed - though one has to include information about the behavior of materials in addition to light. From the theory of radio and microwaves to visible light and beyond - Maxwell's equations provide a deep understanding of light.

    4. Quantum theory of light: but some aspects of the interactions of light and matter could not be explained by the previous theories - hence Planck's quantum explanation for black body radiation, Einstein's explanation for the photo-electric effect, and also his prediction of stimulated emission - the foundation for lasers. The quantum theory of light explains atomic spectra and many other interactions between light and matter. But the connection between Maxwell's equations and the quantum theory are very strong: photons are the quantization of the modes of the electromagnetic field. You cannot really do quantum optics without an understanding of Maxwell's equations.

    Thus we teach all of them - each is useful, and gives correct results within its realm.

    But you cannot study quantum electrodynamics in middle school! However, you can do experimental demonstrations of various optical effects, working your way through all of the theories - but an explanation requires an increasing sophistication of physical understanding along with an ever-increasing mathematical inventory of tools.

    Hence we teach them all, but in some useful order which provides depth of understanding.
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