Shedding Light on X-rays


The 1914 Nobel Prize for Physics united two issues of identity that had been perplexing physicists in the early 1900s. One was understanding the true nature of the mysterious X-rays. The other was how to prove the theory that crystals consist of atoms arranged in a regular lattice structure. Max von Laue received the award for showing that both problems could in fact be solved neatly by a single experiment.

At the time, there was a heated debate as to whether X-rays are highly energetic particles, or waves of electromagnetic radiation, like light. X-rays appeared to behave like light waves in some senses, but not others. For instance, when X-rays passed through a series of slits, they didn’t appear to behave as light does when it resembles water ripples dispersing through reeds – a process called diffraction and interference.

During a conversation with a colleague about whether shining light through crystals could help reveal their structure, the connection suddenly struck von Laue. If, as was known, diffraction typically occurs when the distance between slits is of a similar size to the wavelength of light, the proposed size of spacing between the layers of atoms in a crystal lattice would be much smaller than the wavelength of visible light. And if, as some physicists had claimed, X-rays had a much shorter wavelength than light, then the spaces between the atoms in a crystal might be just the right size to diffract X-rays.

What seems an experiment based on an improbable set of assumptions produced a successful outcome – a beam of X-rays passing through a crystal formed a characteristic interference pattern of bright spots on a photographic plate. Laue’s ingenious discovery of the diffraction of X-rays in crystals revealed both that X-rays are a form of electromagnetic wave and that crystals are lattice-like in structure. His work provided the spark for using X-rays and crystals to uncover the secrets of chemical compounds at the atomic level.

By Joachim Pietzsch, for


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