Young's Double-Slit Experiment

Coherent light passes through two narrow slits; each slit becomes a fresh source of waves. Where crest meets crest the light adds, where crest meets trough it cancels — producing the fringe pattern on the screen. Students can vary the slit separation d, screen distance D, wavelength λ and slit width a, and switch between a plane wave and a point source, watching the pattern reshape in real time. The fringe spacing β = λD/d is computed exactly.