whatLight arrives one photon at a time, each a single tick on the screen at the bottom, each landing somewhere no one can predict. Out of that randomness, stripes assemble: the interference fringes, built dot by dot. Click to switch on a detector that watches which slit each photon uses — the new arrivals stop striping and pile into two plain bands, while everything already recorded stays. Click again to switch it off and watch the stripes return, layered on top.

whyThis is the two-slit experiment, the one Feynman called the only mystery. Each photon lands randomly, yet the crowd draws a pattern that only exists if each photon somehow took both slits; know the path, and the pattern is gone. The piece keeps the whole history on screen, so the eras with and without the detector sit stacked in the same ink.

howEach photon's landing place is drawn at random from the squared wave amplitude: with no detector, the two-slit distribution, a fine cosine ruled under a soft single-slit envelope; with the detector on, the interference term is removed and only the two envelopes remain. Nothing else changes. Arrivals stack as tick columns, so the histogram is the picture; measured photons also flash their path through the slit that saw them, unmeasured ones arrive with no story to draw. The readout counts photons and tracks the fringe visibility of the last few hundred arrivals, which collapses when the detector goes on and recovers when it goes off. One integer seeds the randomness.