Fig 3Ripple tankMove across for f · up and down for spacingf 16.0 Hz · λ 1.45 cm · d 5.0 cm · 6 nodal lines
Two dippers, in phase: the sourcesTank on legs, bridge and vibrating barThe paper, 21.2 cm down: the pattern 1.53× life sizeA ripple tank seen by strobe light. 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, which is honest slow motion. 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 and a crest travels 23 cm/s. Wherever the paths from the two dippers differ by half a wavelength the waves cancel, so dippers d apart draw 2⌊d/λ + ½⌋ nodal lines: 6 at 5 cm. A crest one wavelength out stands only 0.069 mm high (ka = 0.03), yet it is a weak lens. The lamp is 40.0 cm above the water and the paper 21.2 cm below it, so the shadow is 1.53 times life size and the paper sits 0.59 of a crest's focal length down at 16 Hz, 0.94 at 28 Hz. Near the dippers, where the two waves add, the light passes through focus and every crest draws a sharp bright line, and the plain grey bands that break them are the nodal lines. A sloped foam beach inside the walls soaks up each wave before it can reflect. The damping here is clean water's viscosity alone, so these ripples carry farther than in a real tank, where a surface film calms them sooner.