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The Davisson-Germer experiment (1927) scattered low-energy electrons from a nickel crystal and observed strong diffraction peaks that followed Bragg's law nλ = 2d sinθ. The observed wavelengths exactly matched de Broglie's prediction λ = h/p, providing the first direct confirmation of matter waves and a landmark proof of wave-particle duality.
Study the original experimental setup and data. Calculate expected wavelengths for the electron energies used and verify Bragg's law. Understand why the diffraction pattern requires the electron to have wave-like properties.
The Davisson-Germer result is not explained by assuming electrons are classical particles scattering randomly (the sharp diffraction peaks rule this out). The experiment requires electron momentum to be de Broglie wavelength.
From your study of electron diffraction and the de Broglie hypothesis, you know that matter has a wavelength λ = h/p. This was a bold theoretical prediction in 1924, but the question physicists urgently needed answered was: is this real? Does matter actually exhibit wave interference, or is the wavelength just a mathematical convenience with no observable consequences? The Davisson-Germer experiment answered this question with a resounding yes, and did so in a way that left no room for classical alternatives.
The experimental setup was straightforward: a beam of low-energy electrons (accelerated through a few tens of volts) was aimed at a nickel crystal, and a movable detector measured the number of electrons scattered into each angle. The crystal was not chosen arbitrarily — its regularly spaced atomic planes act as a diffraction grating for anything with the right wavelength. Bragg's law, nλ = 2d sinθ, describes the constructive interference condition: waves reflect from successive atomic planes and reinforce only when the path length difference between them is a whole number of wavelengths. The nickel crystal's lattice spacing d was already known from X-ray crystallography.
The result was unambiguous: the scattered electron intensity peaked sharply at specific angles exactly matching Bragg's law. When Davisson and Germer computed the wavelength that would produce peaks at those angles, they got λ ≈ h/p for electrons of the measured energy — precisely de Broglie's prediction. At 54 eV, the electron momentum is p = √(2mE) ≈ 4.0 × 10⁻²⁴ kg·m/s, giving λ = h/p ≈ 0.166 nm. The nickel crystal spacing is about 0.215 nm, and the observed diffraction peak at 50° is consistent with this wavelength and spacing via Bragg's law. The match was not approximate — it was quantitative agreement at the percent level.
What rules out a classical explanation? Classical particles bouncing off a crystal surface would scatter in a broad, diffuse pattern — some atoms would scatter more, others less, but there would be no sharp constructive interference peaks. The existence of peaks at specific angles, with dark regions in between, is the hallmark of wave interference. Just as two water waves cancel where trough meets crest and reinforce where crest meets crest, the electron "waves" scattered from successive crystal planes cancel at most angles and reinforce only where the Bragg condition is satisfied. This is the same phenomenon that makes soap bubbles iridescent and explains why X-rays diffract from crystals. Davisson-Germer placed electrons in the same category: real waves, not a mathematical fiction. This result, alongside G.P. Thomson's independent electron diffraction experiment using thin metal foils, established wave-particle duality as an experimental fact about matter itself.