• summary_type: concept
• word_count: 150

Three key experiments validate quantum theory: (1) Photoelectric effect (Hertz 1887, Einstein 1905, Millikan 1916): light behaves as particles (photons). The $V_{0}$-f graph slope gives h/e, confirming quantization. (2) Davisson-Germer experiment (1927): electrons show diffraction from nickel crystals, confirming matter waves. 54 V electrons gave λ = 1.67 Å matching de Broglie's prediction. G.P. Thomson independently confirmed electron diffraction through thin metal foils. (3) Compton scattering (1923): X-ray photons scatter off free electrons with wavelength shift $\Delta$λ = (h/mc)(1-cosθ), confirming photon momentum. These experiments collectively established wave-particle duality as a fundamental property of nature. Bohr's complementarity principle unifies them: wave and particle properties are complementary, never manifesting simultaneously in the same experiment.