Each photon lands as a single dot. Yet thousands of them, sent one at a time, draw stripes that only a wave passing through both slits could make. Then try watching which slit each one uses.
Let a few hundred photons land, then switch on the which-path detector. Changing a setting clears the screen so patterns don't mix.
A source sends single photons toward a barrier with two narrow slits, one metre in front of a screen. Each photon arrives at one random spot. The odds of each spot follow the wave formula: two waves, one from each slit, add up where their crests meet and cancel where a crest meets a trough.
With the detector on, every photon is tagged with the slit it went through. The stripes disappear and you get two overlapping blobs, exactly what tiny bullets would make.
No photon is ever split; detectors always click whole. But whenever the universe holds no record of which slit was used, the photon behaves as if it explored both. Gaining that record, even without disturbing the photon's direction, destroys the interference. This is wave-particle duality, and Feynman called it "the only mystery" of quantum mechanics.
Done for real with single electrons by Tonomura's team at Hitachi in 1989.
The stripes on your screen are not just a curiosity. Turning interference patterns back into distances, and using matter as a wave, is behind some of our sharpest instruments.
A crystal is millions of regularly spaced "slits". X-rays scattered off it form a pattern of spots, and the fringe-spacing rule run in reverse gives the spacing of the atoms. Rosalind Franklin's Photo 51 revealed DNA's double helix this way, and most known protein structures came from it.
In the demo: spacing = wavelength × distance ÷ separation, solved for the separation.Electrons interfere just like photons, with a wavelength set by their speed. At typical microscope voltages that wavelength is about 100,000 times shorter than visible light's, so electron microscopes can resolve individual atoms and the parts of viruses.
In the demo: a shorter wavelength squeezes the stripes and sharpens detail.LIGO splits a laser beam down two 4 km arms and recombines it. A passing gravitational wave stretches one arm and squeezes the other by less than a thousandth of a proton's width, which shifts the interference from dark to faintly bright. It first detected merging black holes in 2015.
In the demo: a tiny change in path length moves a dark fringe.A diffraction grating is a double slit with thousands of slits. Each colour lands at its own angle, spreading light into a sharp spectrum. That is how we read what distant stars are made of, and why a CD shows a rainbow.
In the demo: change the wavelength and watch the fringes move.Diffraction blurs any feature smaller than about the wavelength used to draw it. That limit pushed chipmakers from visible light to deep ultraviolet and then to 13.5 nm extreme ultraviolet to print the transistors in today's phones.
In the demo: narrow slits spread light wide; the envelope is diffraction.A hologram is a recorded interference pattern between laser light from an object and a reference beam. Light it again and the pattern rebuilds the original wave, depth included. Embossed versions guard banknotes and credit cards because they are hard to copy.
In the demo: the screen is a record of two waves meeting.