In the same way, x-rays passed through a single crystal produce a pattern of rows of spots. A metal foil will produce a set of diffraction rings instead, because the metal is made up of many tiny crystals oriented at random.

The spacings of the rings tells us how closely the atoms are packed together in the metal. X-ray diffraction by crystalline solids currently is the most accurate method of determining the structures of molecules.

This is the way that waves behave, yet we know that light and x-rays also have particle properties. Nature often is symmetrical and, in retrospect, it was no great flash of insight for physicists to ask themselves, since waves are also like particles, whether entities such as electrons might not also be wavelike.