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Evidence for wave and particle properties of light

  1. Feb 21, 2006 #1
    I am really stuck on this problem.

    What evidence supports the notion that light has wave properties? What evidence supports the view that light has particle properties?

    I know that the Quantum theory tells us that both light and matter consists of tiny particles which have wavelike properties associated with them but I am still unsure what is being asked....Any advice?

  2. jcsd
  3. Feb 21, 2006 #2
    The best evidence for light as a wave is Young's double slit experiment. Shine a monochromatic light beam (a laser works nicely) at two thin slits separated by a large distance (relative to the slit size). Projected onto a screen in the background we get a diffraction pattern...a hallmark of waves.

    The best evidence (I believe) for light as a particle is either Compton scattering or the photo-electric effect. In Compton scattering an x-ray is scattered off a free electron; the results imply a particle collision. In the photo-electric effect light is introduced to a metal surface, which then ejects electrons. The manner in which the electrons are ejected (the energy of the electrons depend on the frequency of the light, not the intensity) also imply a particle model.

    Of course, quantum says they are neither. Quantum "particles" have either wave-like or particle-like properties depending on which experiment we do. Any experiment will detect one type of property or the other, but not both. If that confuses you, you are in good company. No one else really understands it either, almost 100 years after the concept was introduced.

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