5.3 Wave Motion

There are also two other ways to classify waves - by their motion. A wave in which energy is transferred from one place to another as a result of its motion is called a progressive wave. An ultraviolet light wave, which transfers energy from the sun to the skin of people lying on the beach, for instance, is a progressive wave. In general, waves that move from one point to another transfer some kind of energy. In a progressive wave, the shape of the wave itself, is what is transferred, not the actual components of the medium.

This animation of a dog on a leash shows a progressive wave transferring energy from the boy to the dog, which end up getting flipped through the air.

In stationary, or standing waves, the shape, or profile of the wave stays fixed in a medium. An example of a stationary wave is produced on the string of a string instrument. When the string is plucked, a wave is caused to travel up and down it. Since both ends of the string are fixed, the waveform is reflected back up and down the string, or its path, causing a wave to stay constant within it. This is due to a phenomenon known as interference, which will be further discussed below.

When the strings of an instrument are plucked, however, a sound is produced. This is because the energy within the string that is used to create the standing wave is also creating a progressive wave, the sound wave you hear. For a stationary wave to exist indefinitely, no energy loss must occur from the (in this case string) system. In other words, it can only exist in an undamped system, for instance, in a vacuum.

There are special terms for two parts of stationary waves, which become more important when discussing harmonics. Points along the stationary wave where the displacement is zero are called nodes, and points of maximum displacement (in this case it can be either in a negative or positive direction) are called antinodes. Unlike in a progressive wave, where all points move with the same amplitude, each point in a standing wave moves with a different one. All points in between nodes are in phase, where in progressive waves, for points to be in phase, they must be separated by one or more wavelengths (or time periods).