Davisson-Germer experiment

The Davisson-Germer experiment, completed in 1927, gave the first clean, direct proof that electrons behave as waves. Clinton Davisson and Lester Germer fired a beam of electrons at a crystal of nickel and measured how many bounced off at each angle. Instead of scattering uniformly like tiny pellets, the electrons came off in strong peaks at specific angles — exactly the kind of constructive interference a wave produces when it reflects off the regularly spaced rows of atoms in a crystal.

What makes the result so convincing is that it was quantitative. The angles of the peaks matched the predictions of de Broglie's formula precisely: feed in the electrons' momentum, compute the de Broglie wavelength, apply the standard law for diffraction from a crystal lattice, and the observed pattern falls out. The crystal acted as a natural diffraction grating, with atomic spacing close enough to the electron wavelength to do the job.

The discovery was partly a happy accident — a laboratory mishap that recrystallised the nickel into large grains is what sharpened the peaks — but its impact was decisive. Together with independent work by George Thomson, it confirmed matter waves beyond doubt and helped earn Davisson and Thomson the 1937 Nobel Prize. De Broglie's audacious symmetry argument had become hard experimental fact.