Before beginning a formal discussion of the nature of waves, it is often useful to ponder the various encounters and exposures which we have of waves. For many people, the first thought concerning waves conjures up a picture of a wave moving across the surface of an ocean, lake,pond or other body of water. The waves are created by some form of a disturbance, such as a rock thrown into the water, a duck shaking its tail in the water or a boat moving through the water. The water wave has a crest and a trough and travels from one location to another. One crest is often followed by a second crest which is often followed by a third crest.
Every crest is separated by a trough to create an alternating pattern of crests and troughs.
The waves may appear to be plane waves which travel together as a front in a straight-line direction, perhaps towards a sandy shore. These mental pictures of water waves are useful for understanding the nature of a wave and will be revisited later when we begin our formal discussion of the topic. When performed with reasonably good timing, a noticeable ripple is produced which travels around the circular stadium or back and forth across a section of bleachers. The observable ripple results when a group of enthusiastic fans rise up from their seats, swing their arms up high, and then sit back down. The process continues, as each consecutive row becomes involved by a momentary standing up and sitting back down.