Waves move, water stays
Why a wave can cross an ocean without carrying any water with it.
What you are looking at
Each dot moves in a small circle. The pattern across all of the dots makes a wave that travels along.
The idea
Have you seen the "wave" at a sports stadium? People stand up and sit down one after another, and the wave goes all the way around. But no person goes around the stadium. Each person only goes up and down.
Water waves work the same way. A cork floating on the water goes in a small circle as a wave goes by, and ends up almost where it began. The wave carries energy across the sea, but it does not carry the water.
The dots in the animation each go in a small circle. Because each dot is a little bit behind its neighbor in its circle, you see a wave shape move across the screen.
Where else you see it
- A buoy bobbing in the sea.
- A rope that you flick: the wave runs along, but the rope stays.
- Sound is a similar idea: the air shakes back and forth, but does not travel along with the sound.
Go deeper: the proper names and the math
In deep water, particles follow near-circular orbits whose radius shrinks with depth like e−kz, so a wave is barely felt at a depth greater than half a wavelength. Wave speed depends on wavelength: c = √(g/k) = √(gλ/2π), so longer waves travel faster (this is called dispersion), which is why distant storms send long swells ahead of shorter waves.
The circles do not close perfectly. There is a small net drift in the direction of the wave (Stokes drift). In shallow water the orbits flatten into ellipses and the waves slow down and steepen as they approach the shore.