Wave Motion



If you look at the sea, you see waves. As you look across the surface, the water goes up and down. The shapes and movements can be quite random or quite regular. They can be big or small. They are always complicated.

To learn about wave motion, we often start with a few simple types: wave in a rope, wave in a spring, and wave in a ripple tank.

Think of a long rope. One end is fixed to a wall. You hold the other end in your hand and walk away from the wall until the rope is roughly horizontal. Now move your hand up and down quickly. This creates a wave-like motion in the rope. It is an alternate up and down pattern along the rope. We may call the higher parts the peaks, and the lower parts the troughs. The peaks are roughly equally spaced, and they move away from you.

This description contains a few features that we often associate with wave motion:

  1. It happens over a stretch of space - in this case along the rope.
  2. There is a pattern that repeats itself over and over, like the peaks and the troughs.
  3. The pattern moves along the space, just as the peaks and the troughs move away from you on the rope.

Next, imagine a long, spiral spring lying on a table. One end of the spring is fixed to an edge of the table. Hold the other end with you hand. Quickly move you hand to and fro along the axis of the spring. This compresses and stretches the spring repeatedly. You will see alternate compressed and stretched regions along the spring. The compressed regions are roughly equally spaced, and they move away from your hand.

This motion of the spring does not have the shape that we normally associate with wave. When we think of a wave, we think of motion with a repeating pattern that moves up and down and that travels horizontally, like on the sea or along the rope. For the spring, the motion is not up and down. Instead, it is horizontal, along the same line that the pattern moves in. However, the spring motion shows the same three features that I listed above for a wave:

  1. It happens over a stretch of space - in this case along the spring.
  2. There is a pattern that repeats itself over and over, like the compressed and stretched regions.
  3. The pattern moves along the space, just as the compressed and stretched regions move away from you on the spring.

From this similarity, we may conclude that the spring motion above fits the description of a wave.

The two different motions of the rope and the spring are actually examples of two different types of wave motion. In the rope, the oscillation (up-down) is perpendicular to the direction that the wave travels (horizontal). We call this a transverse wave. In the spring, the oscillation (horizontal) is parallel to the direction that the wave travels (also horizontal). We call this a longitudinal wave.

Many waves belong to either one of these two types. Light waves are transverse waves. Sound waves are longitudinal waves.

We should mention the ripple tank. This is a large, shallow tank with water, used to study water waves. If you take a pencil and poke it into the surface of the water, you would generate a ripple - circular waves that radiate away from your pencil. If you hold a long ruler horizontally, and push this into the water surface, you would see long, straight wave fronts moving away from the ruler.



Copyright 2010 by Kai Hock. All rights reserved.
Last updated: 15 May 2011.