Draw walls. Watch the waves bend.

A ripple tank in your browser. Two gaps split a wave, the pieces overlap, and bright bands appear on the screen, right where the formula says.

Ripple Tank

Fig. 1. Waves are starting.

A wave solver written from scratch: The tank is a 2D wave equation stepped on the GPU: WebGL2 float textures ping-ponged at 60 frames a second, with walls, shallow plates that slow the wave, and an absorbing edge so nothing bounces back from the glass.
A twin that checks the GPU: A second copy runs on the CPU in a worker, with its own sine and Float32 arithmetic. At start-up it checks the GPU against it and takes over if they disagree, so the tank never goes blank.
Shadowgraph rendering: Brightness comes from the curvature of the water surface, the way a real tank's lamp projects it: crests that curve up focus light, troughs spread it.
Measured, not painted: The bright bands on the screen are found by fitting a peak to the averaged brightness and compared with the textbook formula d sin θ = mλ in the caption.
Next: a daily puzzle: Daily Focus is planned: a physics puzzle scored on Pages Functions and D1 from a table the CPU twin precomputes, so the server never simulates.

Fig. 2. The next height at a point is twice its height now, minus its height one step ago, plus the pull of its four neighbours.