Double-Slit Experiment

Thomas Young shone light through two closely spaced slits in 1801 and showed the light landing on a screen beyond formed alternating bright and dark bands, a pattern that only makes sense if light waves passing through the two slits interfere with each other, and his result argued strongly for light as a wave rather than a stream of particles. The experiment was repeated in the twentieth century sending single electrons or photons through one at a time, and even individual particles, arriving one by one, build up the same interference pattern over time. Placing a detector at the slits to learn which one a particle passed through destroys the pattern completely, a result that shows particles behave as waves except when measured directly, the demonstration now used to introduce wave-particle duality. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/