General Properties Of Waves

Drop a pebble into a still pond and something strange happens. Rings race outwards across the surface, faster than you could swim, and a leaf floating two metres away lifts, tips and settles back down. The leaf does not travel. It ends the afternoon exactly where it began. Yet something crossed those two metres of water, because the leaf moved and it was the pebble that moved it. Whatever made that journey carried no water with it at all.

That is the idea this lesson is built on, and it is one of the most useful in the whole of physics. The same description covers the ripple on the pond, the note from a guitar, the light reaching your eye from this page and the signal your phone pulled out of the air a moment ago. You will learn what a wave actually is and what it carries, the five words physicists use to measure one, the single equation that ties speed, frequency and wavelength together, and what happens when a wave stops having a clear run and arrives at the edge of a different material. By the end you will be able to work out the speed of a ripple from a stopwatch and a ruler, and to say why you can see your own reflection in a window at night while the person inside is reading comfortably by the very lamp you are looking past.