Photons through two slits. The interference pattern. Wave-particle duality made interactive.
In 1801, Thomas Young took a candle, made a small hole in a card, placed a second card with two narrow vertical slits behind it, and put a screen behind that. The light fell on the screen as bright and dark bands. Young called the bands fringes, did the geometry, and concluded that light is a wave. He was right. He was also ignored. Newton had said light was particles a century earlier, and disagreeing with Newton in the early 1800s was not a thing English physicists did.
A hundred years later the experiment came back stranger. By the 1920s, de Broglie had said matter is also a wave. By the 1960s, experimenters could fire one electron at a time at a double slit and watch the dots accumulate on a phosphor screen. Each electron arrived as a point. But the dots, over thousands of arrivals, built the same fringes that Young saw with light. The experiment was repeated with neutrons. With whole atoms. With molecules the size of buckyballs. The pattern held every time.
Then came the question that broke physics for a generation: which slit does each particle go through? The experiment was set up so the measurement could be made. A detector at one of the slits — even a passive one, even one whose data nobody read — was enough. The fringes vanished. The particles arrived as two single-slit patterns, the same way bullets would. The wave behavior disappeared the moment the question was asked.
That is the experiment in this lab. Run it as a wave. Run it as particles. Then turn on the detector and watch the wave behavior leave.
"We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery."
The earlier labs in this suite — Chladni, Acoustic Levitation, Ripple Tank — all teach transparent replication. The mechanism is visible. You toggle a parameter, watch cause produce effect, reach a documented result. You leave knowing what is known.
This lab is the first in the suite that teaches discovery under uncertainty. The dominant factor is hidden. The student fires photons and watches a pattern emerge, and has to decide what the pattern means before anyone tells them. The lesson is not the answer to the wave-particle question. The lesson is that the wave-particle question, as a question, is malformed — the particle does not have a path when you don't measure, and the moment you measure, it picks one. Reality, at this scale, is not a thing you look at. It is a thing you make by looking.
WAVE. The continuous wave passes through both slits and lays down classical fringes. Bright bands at the maxima, dead bands at the minima. Same math as Ripple Tank's double-slit mode.
PHOTON RAIN. Now the source fires single particles. Each lands as a dot. The dots are individual. The pattern, however, is the same as the wave. Each photon — alone — somehow passes through both slits, even though "both" should not be a thing a single particle can do.
WHICH-SLIT. A detector is turned on. Each photon's path is observed. The fringes collapse to two single-slit blobs — the incoherent sum of two separate slit patterns. Measuring which path destroyed the interference. The photon stopped being a wave the moment we asked.
ONE SLIT. The control. Close one slit. The remaining pattern is just the smooth single-slit envelope. Compare it to WHICH-SLIT mode — the two are the same math, two times. Compare it to WAVE — they are totally different. The fringes are not a property of light. They are a property of having two paths available simultaneously.
Two physics engines run side by side. The WAVE mode runs the same 2D wave equation as the Ripple Tank lab, with a barrier and two slits cut into it. The PHOTON RAIN, WHICH-SLIT, and ONE SLIT modes sample real photon arrival positions from the Born-rule probability distribution P(y) = |ψ1(y) + ψ2(y)|2 when the slits are coherent, or the incoherent sum P(y) = |ψ1(y)|2 + |ψ2(y)|2 when the which-slit detector is on. Each dot on the screen is a real draw from that distribution — not a visual sparkle.
The pedagogy is faithful. The numerical values are dimensionless. Wavelength, slit separation, and slit width are all in canvas pixels — not nanometers, not micrometers. The instrument teaches the shape of the phenomenon, the same standing rule the other suite labs follow.
Section 4.9.4d · Suite Lab 4 of the OPA Browser Physics Lab Suite · First discovery-under-uncertainty lab in the suite · Filed under Building 9 (Stephens Science Center), College IX (Science), Opathorlokan University, Birmingham, Alabama.
Built by Travis Jenkins (User Zero) with Claude. The instrument exists so a student can meet the wave-particle question with their own hands before anyone tells them what the answer is.