Surface wave patterns from point sources — interference, diffraction, and refraction in a shallow basin.
A ripple tank is the simplest possible piece of physics teaching equipment: a shallow glass tray of water, a strong light from above, and a screen below. When a finger or a paddle dips into the water, ripples spread outward. The light casts crests as bright lines and troughs as dark ones onto the screen, and a wave that would otherwise be too quick and too small to study becomes a slow, enormous shadow on the wall.
The instrument has no single inventor. By the late 1800s, physicists like Humphrey Lloyd in Dublin and Heinrich Hertz in Karlsruhe were doing optical-bench experiments โ interference fringes, standing waves, diffraction โ and reaching for water analogies whenever a class needed to see the math. A ripple tank with a stroboscopic lamp does that job almost too well. The bright bands and dark bands you would see in a Young's double-slit experiment with light show up at the size of your hand when you do the same experiment with water.
By the 1950s, every secondary school physics lab in the English-speaking world had one. They were the way an entire generation of students first met the words diffraction, interference, and refraction. The tanks are still being built, but the cost, the leaks, and the strobe lamp have always kept them from reaching most classrooms. A browser version doesn't replace the real thing โ it makes the lesson reachable when the tank can't be.
The instrument did not become important until Thomas Young passed light through two narrow slits in 1801 and showed that the screen behind them was striped with bright and dark bands. Light, the bands proved, was a wave. The ripple tank made that argument visible in water decades before any classroom could afford a laser, and it still does.
The water on the screen above is the gateway lab for all of classical wave physics. Once a student has seen ripples bend through a slit, the same student knows what light is doing in a Young's experiment, what sound is doing in a concert hall, what a seismic wave is doing as it crosses a soil boundary, and what ocean swell is doing as it refracts into a bay. The math is the same; only the medium changes.
Propagation (Huygens). A pulse spreads as a circle because every point on the wavefront is itself a tiny new source. Drop a pulse in calm water and watch the geometry: the ring stays a ring until something interrupts it.
Diffraction. Push a wavefront at a barrier with a single narrow gap and the wave does not stay in a straight beam โ it fans out from the gap. The narrower the gap, the more the wave spreads. This is the lesson that cost optical physics decades to accept: waves bend around corners.
Interference. Push the same wavefront through two gaps and the two emerging waves meet on the far side. Where crest meets crest they reinforce; where crest meets trough they cancel. The pattern of bright and dead bands fans out. That pattern is the entire reason we know light is a wave.
Refraction. Slow a wave down by sending it into shallower water and it bends โ the wavefront pivots toward the slower side. Same math as light bending as it enters glass. Snell's law is just a label on this lesson.
A browser-based 2D wave equation solver, drawn at 240×160 grid resolution and upscaled to the canvas. The math is finite-difference on utt = c2∇2u with a CFL number of 0.5, edge-damping ramps to absorb outgoing waves, and per-cell wave speed so the shallow-wedge mode can refract the wavefront. Walls are zero-amplitude hard constraints.
The lessons (Huygens, diffraction, interference, refraction, reflection) are physically faithful. The numerical values (wavelength in canvas units, wave speed in canvas units per second) are not matched to any specific physical tank. The instrument teaches the shape of wave behavior, not a lab-grade calibration to a particular apparatus. That honesty is the suite's standing rule, the same one carried by the Chladni Plate and Acoustic Levitation labs.
Section 4.9.4c · Suite Lab 3 of the OPA Browser Physics Lab Suite · Filed under Building 9 (Stephens Science Center), College IX (Science), Opathorlokan University, 900 Arkadelphia Road, Birmingham, Alabama 35254.
Built by Travis Jenkins (User Zero) with Claude. The instrument exists because the lesson needed to be reachable when the tank can't be.