Gerjuoy, Edward Department of Physics, University of Pittsburgh, Pittsburgh, Pennsylvania.
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The modern theory of matter holding that elementary particles (such as electrons, protons, and neutrons) have wavelike properties. By 1915, experiments on the diffraction (bending) of x-rays into special directions by crystals had established that x-rays were electromagnetic waves, akin to visible and infrared light, but of much shorter wavelength than those other electromagnetic radiations. However, in 1923 A. H. Compton showed that observations on x-rays scattered by, for example, a graphite target could be quantitatively predicted via the hypothesis that the x-ray scattering from each individual target atom resulted from elastic (billiard-ball-like) collisions between the comparatively slowly moving atomic electrons and what were in effect particles (now commonly called photons) in the incident x-radiation, with each incident photon having an energy equal to the product of Planck's constant (6.6 × 10-34 joule second) and the speed of light divided by the wavelength, and momentum equal to Planck's constant divided by the wavelength. Thus, some experiments with electromagnetic radiation seemingly can be understood only by visualizing the radiation as waves, while other experiments on the same radiation seemingly require that the radiation be visualized as a stream of particles. This quite unintuitive wave-particle duality manifested by electromagnetic radiation is all the more remarkable in that the particlelike properties associated with the radiation, namely the energy and momentum of its photons, are given in terms of its wavelength, a concept that seemingly has no meaning except in a wave context. See also: Compton effect; Photon; X-ray diffraction
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