Understanding wave-particle duality

One of the most confusing concepts in physics, wave-particle duality is unlike anything we see in the ordinary world.

Light is a wave. This fact is something that most people are familiar with and it’s been known since the experiments carried out by the English polymath Thomas Young in the early 1800s:

Thomas Young’s sketch of two-slit diffraction of waves, 1803

When he wasn’t busy helping to translate the hieroglyphs on the Rosetta Stone, investigating elasticity, describing how colour vision works, or proposing a new universal phonetic alphabet, Young was experimenting with the diffraction of light by narrow apertures and the interference of light due to reflection by thin films.

His observations of coloured fringes in these experiments demonstrated that light must travel from place to place in the form of a wave, and the different colours comprising white light could be characterized by different wavelengths.

Engraving of James Clerk Maxwell

Such work culminated half a century later with the research of the equally brilliant Scottish physicist, James Clerk Maxwell. He calculated that oscillating electric and magnetic fields would propagate through space with the speed of light, and that therefore light itself was an example of electromagnetic radiation.

It was subsequently recognised that a whole spectrum of electromagnetic radiation exists, from the short wavelength gamma-rays and X-rays at one extreme, through the more familiar regions of ultraviolet radiation, visible light and infrared radiation, to the long wavelength phenomena of microwaves and radio waves at the other.

The wavelength and frequency of each type of electromagnetic radiation are related by the simple equation: wavelength x frequency = speed of light, about 300,000 km/s.

This recognition of light as a wave phenomenon had in fact overturned two centuries of belief, dating back to work by Isaac Newton, that light was composed of a stream of particles. However, light had one more surprise in store.

The theory of quantum physics was derived in the early 1900s, partly to explain the odd behaviour seen in the so called photoelectric effect. It was observed that when light fell on a metal surface, electrons could be emitted by the metal.

Changing the intensity of the light caused the number of electrons emitted to change, but the electrons’ range of energies remained the same.

Decreasing the wavelength of the light, however, caused the electrons to be emitted with a higher maximum energy. Conversely, increasing the wavelength of the light beyond a particular cut-off value (specific to each metal), caused the emission of electrons to cease.

The explanation was that light interacts with matter as if it is composed of a stream of particles, now called photons. Each photon carries a specific amount of energy, which is directly related to the frequency used to characterise the propagation of light by the relation: energy of photon = Planck’s constant x frequency.

A wave and a particle!

Planck’s constant was introduced as a new fundamental constant of nature that lies at the heart of all quantum phenomena.

Each type of metal has a minimum energy required to eject electrons, and so requires photons carrying at least that amount of energy to do so. The minimum energy requirement translates into a maximum wavelength limit, using the two equations listed here.

So light, and all other electromagnetic radiation, is now recognised as something rather strange: it propagates as if it is a wave, but interacts with matter as if it is a stream of particles.

In truth, it is neither a wave nor a particle, but it is convenient to describe its behaviour in terms of one or the other, as the situation dictates.

Andrew Norton is a member of the Astronomy Research Group and Professor of Astrophysics Education in the Department of Physical Sciences at The Open University. This article was previously published in September 2013 on OpenLearn. You should subscribe to our newsletter for more free courses, articles, games and videos.