[X-rays] are a kind of wave with properties no wave has any business to have.
In the spring of 1912 two research assistants in Munich directed an x-ray beam through a crystal and found that the beam was reformed into a well-defined interference pattern. The property most characteristic of periodic waves – their ability to interfere – is shared by the x-rays. Max Laue, the man chiefly responsible for the discovery, thought he had found proof that characteristic secondary x-rays from the crystal are periodic waves. But H. A. Lorentz quickly pointed out that impulses should interfere too. He showed, in a tour de force argument, that the accepted square pulse is an impossible representation of x-rays. William Henry Bragg and his son concluded that the interference maxima could, indeed, be due to irregular x-ray pulses. But, as such, x-ray impulses were not different from ordinary white light. They supported this claim with a new technique of crystal analysis fully analogous to ordinary optical spectroscopy.
Crystal diffraction provided a new tool for the analysis of x-rays. Pushed furthest by Henry Moseley and Charles Darwin, the technique soon showed that some x-rays comprise periodic wave trains of great length. The extremely sharp angular resolution of observed x-ray interference maxima indicated beyond doubt that x-rays are no different, except in frequency, from ordinary light. Rutherford soon extended the technique to the y-rays. Not only could one isolate characteristic γ-rays, he believed, one could show, with some effort, that they interfere too.
The successful integration of the new spectroscopy with the Bohr atom came, as had x-ray diffraction, from Sommerfeld's Munich.