In the electron double-slit experiment, individual electrons gradually build an interference pattern.
A source sends electrons toward a barrier containing two narrow slits and records their arrivals on a screen behind it. Even when electrons are released one at a time, the accumulated distribution forms alternating bright and dark bands associated with wave interference.
This result demonstrates wave–particle duality: each detection is localized like a particle, while the overall probability distribution behaves like a wave. If an apparatus determines which slit an electron passes through, the interference pattern disappears.
The experiment is not evidence that electrons are simply tiny classical waves. Quantum mechanics describes probabilities using a wave function, and measurement changes which alternatives can interfere. Versions using light, atoms, molecules, and electrons have made the same central lesson visible in different physical systems.