If light (a wave) carries particle momentum, does matter run the symmetry the other way — does a particle like the electron have a wavelength, and diffract?
▶ Launch the interactive simulationFrom eV = p²/2mₑ for an electron accelerated through V volts, take de Broglie's hypothesis λ = h/p = h/√(2mₑeV). Treat a nickel crystal as a diffraction grating: surface atoms spaced d apart send a first-order peak to the angle φ where d·sin φ = λ. Read λ off each peak angle across eight accelerating voltages and least-squares fit λ against 1/√V.
Planck's constant h = 6.626×10⁻³⁴ J·s — recovered as the SLOPE of a single straight line through the origin (intercept ≈ 0), and it is the SAME h the blackbody and photoelectric experiments give for light: one constant for waves AND particles, the core of wave–particle duality. The de Broglie scaling exponent −½ (fit of log λ vs log V) is the matter-wave fingerprint — a photon of energy eV would give λ ∝ 1/V, exponent −1. Cross-checked against Davisson & Germer's actual 1927 datum: 54 V electrons peak at φ = 50°, λ = 1.65 Å, within ~1% of de Broglie's 1.67 Å. The fifth pillar of early quantum theory, and the exact converse of Compton scattering