 Hi, I'm Edie Sevick at the Australian National University and I'm going to give you an introduction to waves. Waves are easily recognized. Water waves, stadium waves, waves on a string, and slinky waves. But you may or may not know that sound is a wave or that light has properties that are wave-like. An understanding of waves provides us with tools to understand physical phenomena of light and sound. Let's start with water waves. You've no doubt seen waves moving across the surface of an ocean or lake. Small waves are caused by a disturbance as, for example, a droplet of water disrupting a calm water surface. The wave has a crest and a trough that travels from the disturbance outwards. One crest is followed by a second crest followed by a third, a fourth, and so on, and each crest is separated by a trough. This disturbance or wave propagates away. Of course, you can have several drops or a drip rate creating a continuous ring of waves.