Double-Slit WaveThe most famous experiment in physics, as pure waves. A wave hits a wall with two gaps; each gap becomes a fresh source, and where the two sets of ripples meet they reinforce or cancel — laying down a bar code of bright and dark bands on the far screen.Thomas Young ran it with light in 1801 to prove light is a wave, against Newton. The twist came later: fire the light one photon at a time and the striped pattern still builds up, so each photon somehow goes through both slits — the experiment Feynman called the one that holds “the only mystery” of quantum mechanics.