Drop a pebble in a still pond and rings spread out across the water. Shout across a valley and your voice races to the far side. Flick a rope and a hump runs down its length. All of these are waves — and they hide a wonderful secret: a wave is a travelling disturbance that carries energy without carrying stuff along with it.
Watch a duck bobbing on a pond as ripples pass. The wave sweeps outward, but the duck only bobs up and down — it doesn't get swept to the shore. The energy travels; the water stays put. That single idea explains light reaching us from the Sun, your voice crossing a room, and a tsunami crossing an ocean.
Every wave has a height and a length. The amplitude is how far it swings up and down from the middle — that's how much energy it carries. The wavelength is the distance from one crest (top) to the next. Drag the sliders and watch the same wave grow taller or stretch out longer.
Waves come in two flavours. In a transverse wave (like light or a ripple on a rope) the wobble is across the direction of travel. In a longitudinal wave (like sound) the wobble is a squash-and-stretch along the direction of travel. Whichever kind, they all share the same handful of ideas — speed, bouncing, bending — and this strand takes you through them one at a time.
Begin by meeting the two great families in
You have already felt waves at work: light lets you