Plate 37 · Matter
When Two Ripples Meet
Drop two stones into still water and two rings of ripples spread out and meet. They don’t smash into each other; they pass through and carry on. Where they overlap, some spots rise higher, while others hardly move, and lines of calm water appear.
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Rings Moving Out
A stone hits the water and pushes it aside, and a ring of ripples spreads from the spot. The water doesn’t flow outward with it: each bit of water only goes round in a small circle. What moves out is the wave.
Look closely and you’ll see the little crests inside a ring run faster than the ring itself: each one appears at the ring’s inner edge, runs forward through it and fades out at the front. Waves this short are pulled back by gravity and by the water’s surface tension together; only ripples shorter than about 1.7 cm are pulled back mostly by surface tension.
A clear, shallow pool · the bright lines on the bottom are sunlight gathered by the water’s surface
Passing Through
Drop a second stone and the two rings meet. They don’t bounce off like two balls; they pass through each other and come out exactly as they went in.
Where they overlap, the height of the water is the two waves’ heights added together: crest on crest makes a higher crest, crest on trough cancels out.
Lines of Calm
Tap the water with two fingers together, again and again, so both send out ripples all the time, and lines of almost motionless water appear.
Along those lines, the waves from the two fingers always arrive one as a crest and the other as a trough, and cancel. Between the lines, the two arrive in step and the water heaves most. This pattern is called interference.
Bright Lines on the Bottom
The shimmering bright lines on the bottom of the pool are sunlight, bent as it passes through the rippled surface and gathered together.
The ripple tanks in physics classrooms use this trick: a lamp shines down and the waves’ pattern shows up clearly underneath. Light is a wave too: send the light from one lamp through two narrow slits, and where the two parts meet they draw the same kind of bright and dark bands.
Your Turn
Change how fast the fingers tap and how far apart they are, and watch whether the lines of calm multiply or thin out. Then put the fingers half a beat out of step and see what happens.
Try thisSwitch on “Half a beat apart”: what happens on the line halfway between the fingers?
What the model leaves out
The ripples are capillary-gravity waves in deep water, with the surface tension of clean water at 20°C. Real ripples slowly die away and bounce back off the edges of a pool; here they just fade, without reflections. Wave heights are exaggerated so the motion shows, and the bright lines on the bottom are worked out approximately from the shape of the surface. The waves are shown at a quarter of their real speed; the numbers are real.
A common misreading
When waves move out across water, the water itself doesn’t flow outward: a leaf floating on the surface rides round in small circles, up and forward, then down and back, and ends up almost where it started; the waves carry it along only a tiny bit. A wave carries a shape and energy, not the water. And where two waves cancel, the water hasn’t vanished; the pushes and pulls on that spot simply balance.
Sources
Follow the question
Plate 35The V Behind a ShipA big ship, a small boat, even a swimming duck: the V-shaped wake behind them opens at almost exactly the same angle, about 19.5° on each side.
Plate 15How Sound Crosses the AirSound is a push passed along through the air. The air doesn’t travel to you; the push does.
