The dippers: two balls on one vibrating barWater and its caustics on the paper, drawn by shaders from the same modelA ripple tank seen by strobe light, as a three.js scene with two shader surfaces. Two dippers on one vibrating bar touch 6 mm of water in phase; the lamp flashes 0.75 times a second slower than they dip, so each wave seems to crawl outward at 0.75 Hz. Ripples this small are held flat by surface tension as well as gravity, ω² = (gk + σk³/ρ) tanh kh, so at 16 Hz the wavelength is 1.45 cm. Dippers d apart draw 2⌊d/λ + ½⌋ nodal lines: 6 at 5 cm. The lamp is 40.0 cm above the water and the paper 21.1 cm below it, so the shadow is 1.53 times life size; each crest is a weak lens, and the paper sits 0.59 of a crest's focal length down at 16 Hz, 0.94 at 28 Hz.