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Tufts OpenCourseware
Author: Fulcrum Institute Development Team

On the nature of light and other electromagnetic radiation

by Judah Schwartz

There is a long history of debate about the nature of light (both visible light and light that is beyond the ability of our visual apparatus to discern). In the 1600’s Huygens proposed that light is essentially a wave phenomenon while Newton argued for a “corpuscular theory of light” in which he asserted that light consisted of particles. He was able to explain such phenomena as reflection and refraction and his view was accepted for more than one hundred years.

In the 1800’s a series of experiments were done which seemed to argue strongly that light was wavelike in nature. Towards the end of the nineteenth century Maxwell’s theory of Electromagnetic Radiation seemed to settle the case – all of the known phenomena involving light seemed to be explained by his theory of electromagnetic radiation (both visible and not) carrying energy in waves.

But there is a problem. Suppose we consider what we know about say sound waves. Sound waves need a medium in which to travel. They cannot travel in vacuum because they consist of alternating moving regions of higher and lower density of the material through which they are traveling – no material, no sound.

Is there a “material” that carries electromagnetic radiation as a wave? People tried desperately to invent a “material” called ether that would serve as the material through which the radiation propagated. But no one could prove the existence of an ether and experiments to show its effects always turned out to fail.

So if we are to think of electromagnetic radiation as a wave, what is it that corresponds to the alternating moving regions of higher and lower density in sound waves? Electromagnetic radiation can exert forces on particles that carry charges (like electrons) and if we could take a snapshot of an electromagnetic wave in space we would find that there are regions in which the force on a charged particle would be in one direction and regions in which it would be in the opposite direction. These regions travel in the direction of the motion of the wave with the velocity of light. The distance between a region where is force is up (say) and the adjacent region where the force is also up is called the wavelength of the electromagnetic wave.

Our eyes are perfectly good detectors of a range of wavelengths of electromagnetic radiation from about 400 billionths of a meter which we perceive as a deep violet light to about 700 billionths of a meter which we perceive as red light. As we have become increasingly aware in recent years, our skin tends to be sensitive to and damaged by wavelengths shorter than 400 billionths of a meter.

So far, so good. Then around the beginning of the twentieth century there were a series of experiments whose results seemed inconsistent with Maxwell and with the wave nature of electromagnetic radiation. Specifically, it was found that electromagnetic waves, unlike sound waves and water waves, could propagate in vacuum, without a medium. This went counter to the experience that physicists had had up until that time, in which propagation of waves always involved a medium. Moreover, the classical wave theory predicted unambiguous and incorrect results for the total amount of radiation emitted as an object became hotter and hotter. Planck and Einstein produced explanations of some of these strange phenomena but at the “price” of considering electromagnetic radiation not as waves but as particles carrying energy. We call these particles “photons”. [It should be noted that Planck never really accepted the implications of his finding – he continued to think of these “particles” as a kind of mathematical trick.]

But when we think of particles we normally think of them as bundles of mass. Photons don’t have a mass they do have energy. What could it possibly mean to have a particle with no mass but with energy? Do such particles have momentum and what could the wavelength of such a particle possibly mean?

It turns out that photons do have momentum and do have energy but that the relationship between the energy of a photon and its momentum is not quite the same as the relationship between the energy and the momentum of a thrown baseball. It also turns out that we can associate a “wavelength” with a photon – the lower the energy of the photon, the longer its wavelength.

Among other things this means that a body that emits electromagnetic radiation at many different wavelengths – or put differently, photons at many different energies – can behave in an unintuitive way. It can emit a large number of relatively low energy photons and at the same time a smaller number of higher energy photons.

These are a few of the ways that the physics of the twentieth century helped to resolve some of these “contradictions” between electromagnetic radiation as waves and electromagnetic radiation as particles. There is a body of underlying theory that can explain and predict when electromagnetic radiation will appear to behave as waves and when it will appear to behave as particles. The predictions of this theory are quite specific and have been verified to great precision.

What we seem to have lost is a nice comfortable mental picture of what light is. Sometimes it behaves like a wave, sometimes like a particle. So what is it, really? It may well be that there is no answer that corresponds to our everyday experiences – the best we can do is to think of it as a wave when that works best for us and to think of it as a stream of particles when that works best for us.

[For those of you who are interested you can find interesting further reading in a freely downloadable online physics text called The Modern Revolution in Physics by Benjamin Crowell" In particular look at Chapter 3 Light as a Particle. (You will note that the matter does not end there – Chapter 4 is called Matter as a Wave)]