Double-Slit Interference

Coherent wavefronts stream from a point source and strike a barrier pierced by two narrow slits, exactly as in Thomas Young's famous 1801 experiment. Each slit re-radiates the wave as a fresh circular wavefront, and where the two overlapping wave trains arrive in step they reinforce into bright fringes, while where they arrive out of step they cancel into darkness. Individual "photon" hits are sampled probabilistically from this interference intensity and accumulate on the detector screen at the back, gradually revealing the telltale striped pattern even though each hit lands one at a time. This is one of the clearest demonstrations of wave-particle duality: light (and matter) behaves like a spreading wave while still being detected as discrete quantized events. Watch the concentric ripples spreading from each slit and notice how the buildup on the screen traces out alternating light and dark bands rather than two simple blobs behind the slits.

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