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How the compton effect demonstrated that the light cannot be explained purely as a wave phenomenon

By Abdulla Ibrahim. Created 15/05/2015. Last edition 16/05/2015.

Background


Wave–particle duality is an ongoing conundrum in modern physics. Most physicists accept wave-particle duality as the best explanation for a broad range of observed phenomena; however, it is not without controversy.

This article discuss how the compton effect proved that the light could be observed as  particles and that the wave theory has failed in explaining the behaviour of light in all manners.



Introduction


The duality about the nature of light has been there for a long time, but the ideas about the nature of light has been the topic for discussion even before that duality existed. Aristotle was one of the first to publicly hypothesize about the nature of light, proposing that light is a disturbance in the element aether (that is, it is a wave-like phenomenon). On the other hand, Democritus—the original atomist—argued that all things in the universe, including light, are composed of indivisible sub-components (light being some form of solar atoms).

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The modern duality about the nature of light starts from Huygens - Newton duality about the nature of light, when Christiaan Huygens and Isaac Newton proposed competing theories of light: light was thought either to consist of waves (Huygens) or of particles (Newton).


 

Huygens and Newton


Newton was interested in light from very early on in his career, the work that first brought him to the attention of the scientific community was his experimental investigation of colour, and his invention of the ‘Newtonian’ reflecting telescope (published in 1672). However this work provided no theory of how light worked, and Newton made attempts at this for many years. For various reasons he favoured a particle theory of light – the explanation of light propagating in straight lines, except at interfaces, was then easily understood. Still, light particles were acted upon by an invisible aether.

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He gave the first theory of wave propagation, showing, among other things how they could be built up from ‘elementary wavelets’, radiated in circular patterns from multiple sources.



 

Compton


Although Max Planck and Albert Einstein postulated that light could behave as both a wave and a particle, it was Arthur Compton who finally proved that this was possible. His experiment involved scattering photons off electrons and offered proof for what we now refer to as the Compton effect.

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The compton's original experiment made use of molybdenum K-alpha x-rays, which have a wavelength of 0.0709 nm. These were scattered from a block of carbon and observed at different angles with a Bragg spectrometer. The spectrometer consists of a rotating framework with a calcite crystal to diffract the x-rays and an ionization chamber for detection of the x-rays. Since the spacing of the crystal planes in calcite is known, the angle of diffraction gives an accurate measure of the wavelength.

Image Modified

Figure 1. Compot Experiment.

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The frequency shift will depend on the angle of scattering, and can be calculated from kinematics. Consider an incoming photon of energy hv and momentum hv/c scattering from any electron of mass m. p is the momentum of the electron after scattering and hv', hv'/c are the energy and momentum of the scattered photon.
For momentum conservation, the three vectors hv/c, hv'/c and p must lie in the same plane.



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Figure 2. Compton Scattering.

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After the observation of the compton effect it was clear to scientist that light could be observed in both ways (as particles and as waves).



Reference:


  1. Compton, Arthur H. “ A Quantum Theory Of the Scattering of X-Rays by Light Elements.” Physical Review Phys. Rev. 21, no. 5 (1923): 483–502. doi:10.1103/physrev.21.483 

  2. Dr. James E. Parks. “The Compton Effect-- Compton Scattering and Gamma Ray Spectroscopy” Department of Physics and Astronomy 401 Nielsen Physics Building The University of Tennessee Knoxville, Tennessee 37996-1200. Revision 3.00 (January 6, 2015).

  3. Shailesh R. Kadakia, “Revolution; Light is a wave: Revisiting the outcome of light’s particle nature experiments”: 13-18.

  4. “Compton Scattering.” Wikipedia. Wikimedia Foundation. Accessed May 15, 2015. http://en.wikipedia.org/wiki/compton_scattering.

  5. “Light.” Wikipedia. Wikimedia Foundation. Accessed May 16, 2015. http://en.wikipedia.org/wiki/light.

  6. “Wave–Particle Duality.” Wikipedia. Wikimedia Foundation. Accessed May 15, 2015. http://en.wikipedia.org/wiki/wave–particle_duality.

  7. “Light.” Wikipedia. Wikimedia Foundation. Accessed May 15, 2015. http://en.wikipedia.org/wiki/light.

  8. “Compton Scattering.” Compton Scattering. Accessed May 15, 2015. http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/comptint.html.

 


By Abdulla Ibrahim. Created 15/05/2015. Last edition 16/05/2015.

Modern Physics, spring 2015