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Wave particle duality

or Is light a wave or a particle?

The apparent dual nature of light is easily demonstrated:

Stand outside in the sun. The shadow your body makes in sunlight suggests that light travels in straight lines from the sun and is blocked by your body. In this, light behaves like a collection of particles fired from the sun. Isaac Newton wrote his treatise on Opticks: "Rays of light are very small bodies emitted from shining substances"

Place two sheets of glass together with a little water between them. With care, you will see fringes. These are formed by the interference of waves. Christian Huygens (1629 - 1695) who was a contemporary of Newton rather the wave model of light. His wavelet construction principle has been used to explain reflection and refraction

In his work, James Clark Maxwell (1831-1879) established a clear picture of light a form of electromagnetic energy. Following Maxwell's equations the light can be seen as electromagnetic waves.

In 1925, our understanding of light seemed to have come to an impasse. Particle theory could explain reflection and refraction, and recent experiments in radiation (such as the radiation from hot bodies and the Compton experiment with X-rays). And wave theory could explain the interference and polarization of light which particle theory could not. Thus simple and sophisticated experiments both indicated that light could be a particle sometimes and a wave at others.

Albert Einstein (1924) expressed the dilemma:

There are therefore now two theories of light, both indispensable, and - as one must admit today in spite of twenty years of tremendous effort on the part of theoretical physicists - without any logical connections.(1)

The dilemma prompted Neils Bohr (1928) to offer his 'complementarity principle': that particle theory and wave theory were equally valid. Scientists should simply chose whichever theory worked better in solving their problem. While it got physics out of its immediate hole, coming from someone as important in modern physics as Bohr, it gained a dominance in physics teaching probably never intended.

Over the succeeding years, the currently accepted solution came in the form of the 'quantised electromagnetic field theory', i.e. 'quantum electrodynamics' (QED). The theory merges particle and wave properties into a unified whole. Despite this, the undergraduate physics of light is still often taught as separate chapters on particles and waves with little or no attempt to give an overall understanding of how this can be so. The complementarity principle is still used in books on optics to justify the use of wave theory to explain interference, polarization, diffraction, etc. The student is then left with the impression either that we do not really understand the true nature of light or that physics is simply a collection of tools for solving problems.

Dick's Editorial

Too often as undergraduates we learn about particles and waves as separate concepts and never get to the next level where these concepts are brought together.

This method of teaching is not exclusive to the teaching of the physics of light. In many fields we learn simple methods first only to abandon them later as we learn more sophisticated methods. In some cases this may be necessary, but as an adult I find this method of learning frustrating and ultimately disappointing.

An obvious example is the Special Theory of Relativity. Here, Einstein introduced the concept of non-Euclidian space-time to explain the constant speed of light. In this, he showed Newton was wrong. The familiar Newtonian mechanics we see in the world around us, in the flights of birds, in falling rocks, and speeding cars, is still a valid approximation when speeds are much lower than the speed of light. When speeds are less than about 10 000 km/s Newtonian mechanics generally works very well. We should therefore learn mechanics by starting with Relativity. Starting with Newton simply shows a fear and lack of understanding of Modern Physics.

In the same way, the physics of light should be taught by starting with the unified theory and showing that particle and wave theories are approximations to this and valid under certain definable conditions. When these conditions are met it is then acceptable to use a particle or wave approximation as appropriate.

Reference: 1. A Einstein, "The Compton Experiment", Appendix 3, in R S Shankland (Ed.), Scientific Papers of Arthur Holly Compton, X-Ray and Other Studies, University of Chicago Press, Chicago (1975).

First published on the web: 15 December 1999.

Authors: Richard Payling & Thomas Nelis