Inquiring Minds

Questions About Physics

Light, Wave or Particle?

Electromagnetic radiation such as visible light or radio waves is often described as electromagnetic waves. Wavelength is used to describe where it falls in the electrmagnetic spectrum. Isn't this technically inaccurate, because light consists of quanta of electromagnetic radiation called photons?

Because photons are discrete particles, they have a certain amount of energy, but not a wavelength because they are not waves. I assume the wave model description is used because under certain common situations, it provides accurate results that agree with observation. Isn't the only time that we would actually observe an oscillating electromagnetic field is when we have a lot of photons that are "in-phase" such as from a laser or a broadcast antenna?

How would you describe how this oscillating field occurs if you describe it using photons and not waves?


The question you have asked is one of the central questions in Quantum Mechanics. In 1801, Thomas Young proved once and for all that light is a wave. Before that, Isaac Newton had claimed that light was really a stream of particles, but he did not have much evidence. Young made a clear case by demonstrating that light interferes with itself. If you have two sources that are in synch (coherent), when the distance from the sources to the obseration point differ by an integer number of wavelengths, then you will have a maximum, because the light from both sources is pointing in the same direction there (constructive interference). If, however, they differ by some integer number of wavelengths plus another half you will have a minimum, no light at the observation point (destructive interference).

Later in the nineteenth century, James Clerk Maxwell formulated the famous "Maxwell's equations" of electricity and magnetism. Using them, one can see that light and radio are basically the same kind of wave, with radio having a much slower frequency. Later it was determined that x-rays were also the same with much higher frequencies.

The twentieth century is when it gets strange. Albert Einstein explained an experiment called the photoelectric effect by realizing that light could only impact in dicrete quanta. At first, it was thought this could not be true because everybody knew light was a wave, but there was and is no other explanation for the experiment. It has been seen many times since that light, especially at high frequencies, often acts like a particle, so what is the answer?

Quantum mechanics tells us that light is both. So if you have light of a certain frequence (call it f), then the total amount of energy that can be deposited by that light is n * h * f. Where h is a contant of nature called Planck's constant and n is an integer that corresponds to the number of photons. Planck's constant is very small and n is usually huge in everyday life, so we don't often notice the grains of light, but they are there.

How do we put this together with Young's experiment in 1801? The answer is that when Young did it, there were so many photons at once, they filled in the interferrence pattern completely. We now know that we can actually do this one photon at a time. It turns out that if we do this, we still get the interference pattern. One photon at a time may hit our detector, but they will only ever hit in areas that would be illuminated if a whole lot of light were present, and no photons will hit where the minima would be. As Paul Dirac famously said, "The photon interferes with itself!"

So in the end, light has both the qualities of a wave and a particle. Which description is "better" depends only on the practical considerations of the situation you are trying to understand. Analyzing a radio station's broadcast pattern with photons is tough, and similarly it is difficult to explain gamma rays from nuclear reactions with waves. However, both can be done. I hope this answers your question adequately. If you need further clarification, just let me know.

Gregory A. Davis graduate student at Fermilab from the University of Rochester

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last modified 6/21/1999