What Is a Standing Wave?

A standing wave is a wave that appears to be vibrating in place rather than moving forward. It is caused by the interaction of two identical waves moving in opposite directions along the same medium. The resulting displacements are twice as great as those caused by a single wave. They do not move along the medium, but seem to stay at set points along the line.

A wave is any energy disturbance that travels from one location to another along a medium. Many materials can serve as mediums. Water is a common example, but waves may also travel through rope, air and many other substances. Earthquakes are caused by seismic waves moving through layers of the planet.

It is important to note that it is the energy, not the medium, that travels in a wave. If two people are standing in a gymnasium holding a jump rope taught between them, one of the people can move the rope up and down, causing a wave to move through the jump rope. The rope moves up and down or side to side, but it does not move across the gym. What is being transported is the energy, not the medium.

A standing wave is formed when energy moves in two directions at once. It is as though both of the people holding the rope started moving it up and down at the same time. Energy travels from each end toward the opposite side. When the energy coming from one side meets the energy coming from the other side, they interfere with each other, causing a standing wave.

The amplitude of a wave is the distance between its highest point and the point of equilibrium. This highest point is called the antinode. In most waves, the antinode appears to move along the length of the medium. With a standing wave, the antinode remains in the same place, because the interference between energies moving in opposite directions always occurs at the same point. This is also true of the nodes, or points of least displacement.

Nodes appear at points where the energy from one side is moving up while the energy from the other side is moving down. The opposing motions effectively cancel each other out. By contrast, the antinodes of a standing wave appear at points where the energy from each side is moving up or down in the same spot. The energy in this case is doubled, making the motion more pronounced. Antinodes are always evenly spaced halfway between nodes.