Detector off — particles interfere with themselves 0 particles fired
Your browser does not support the canvas element. The double-slit experiment shows that single particles, fired one at a time, build up an interference pattern of multiple bands — as if each passed through both slits and interfered with itself. Switching on a detector that records which slit each particle goes through collapses this into two plain bands.
Speed
Detector off. Each particle leaves as a single dot, landing somewhere unpredictable. But over hundreds of runs the dots pile up into bands with gaps between them — an interference pattern. A lone particle, with nothing to interfere with, interferes with itself. It went through both slits at once.
Detector on. The moment something records which slit each particle takes, the spread of possibilities collapses. The same particles now land in just two bands, one behind each slit — exactly what ordinary objects would do. No interference. The looking changed the outcome.