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
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
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.
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
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