Bragg's law for determining crystal structure

Prior to x-ray diffraction, there was almost no way to predict crystal structure. The best that scientists could do is estimate structure based on large scale geometric crystal facets. This changed with the discovery of X-rays. This new radiation could be used to probe materials since its wavelength is near the interatomic separation distance and can therefore undergo constructive or destructive interference. Bragg was clever enough to realize that constructive or destructive interference will be a function of the distance separating different atomic layers. This allows us to calculate interatomic separation by varying in the incoming angle of radiation, theta. This is called Bragg's Law and is now a foundational concept in crystallography.