• Tags: duality, complementarity, philosophy
• Difficulty: Moderate

Wave-particle duality is a fundamental aspect of quantum mechanics. Light shows wave behavior in interference and diffraction experiments, and particle behavior in the photoelectric and Compton effects. Similarly, electrons show particle behavior in cathode ray tubes and wave behavior in diffraction experiments.

Bohr's complementarity principle: the wave and particle aspects are complementary — they never manifest simultaneously in the same experiment. In YDSE with electrons, if we try to determine which slit each electron passes through (particle measurement), the interference pattern disappears (wave behavior is lost). This is the essence of the observer effect in quantum mechanics.

The de Broglie wavelength connects the two: λ = $\frac{h}{p}$ bridges the wave concept (wavelength) with the particle concept (momentum). For macroscopic objects, λ is negligibly small (~10^{-34} m for a 1 kg object at 1 m/s), so wave effects are unobservable.