Electrons are tiny, so a nickel surface should reflect them at various angles since a sample of nickel consists of many randomly oriented small crystals. This is called the diffuse reflection. This is what Davisson and Germer thought in 1927 when directing a low-energy electron beam to a piece of nickel put in a vacuum chamber. The speed of electrons at that beam was about hundred times less than the speed of light. As it often happens, his Majesty an Accident has intervened in their experiment. The sample was accidentally oxidized over its surface, which is not well for reflection. To get rid of the oxide, the scientists had to heat it. As a side effect of heating, the polycrystalline structure of nickel turned into larger crystals. Reflection of the incident electron beam from the surface of that large crystals gave an unexpected picture with peaks of intensity due to interference. The interference picture proved that electrons demonstrate wave-like behavior. Wavelength of such a particle wave is thousands times shorter than that of light so of the order of the period of crystal lattices in solids, that is about 10 nano meters. That crucial experiment had been inspired by the de Brogle’s suggestion that massive particles have wave’s behavior by analogy with electromagnetic waves of light comprised of a number of mass-less particles called photons.
This image is courtesy of monomol.com