Friday, January 7, 2011

Types of waves

Every sound we hear, every photon of light that hits our eyes, the movement of grass blown by the wind and the regular beat of the tides are all examples of waves. They are all around us.  Visible, physical waves such as those we see when a rock is thrown into water are what many people think about when they first began to think about waves. These waves have distinct properties specific to their type but also exhibit characteristics in common with more abstract waves such as sound waves and light (electromagnetic) waves.

At the simplest level, waves are a disturbances that propagate energy through a medium. Propagation of the energy depend on interactions between the particles that make up the medium. Particles move as the waves pass through but there is no net motion of particles.  This means, once a wave has passed the particles return to their original position.  As a result it is energy, not matter, is propagated by waves.

Different types of waves exhibits specific characteristics. These characteristics are used to distinguish between wave types.  Orientation of particle motion relative to the direction of energy propagation is one way waves are characterized.  There are three categories:
  • Longitudinal waves - Movement of the particles are parallel to the motion of the energy.  Sound waves moving through the air is an example of this type of wave.
  • Transverse waves - movement of the particles are at right angles (perpendicular) to the motion of the energy.  Movement of a wave through a solid object like a stretched rope or a trampoline is an example of this type of wave.
  • Surface waves - particles travel in a circular motion.  These waves occur at interfaces.  Examples include waves in the ocean and ripples in a cup of water.  One consequence of occurring at an interface is that the motion of the particles diminish with distance from the interface.  The further from the interface the smaller the rotation of the particles until as some distance from the surface, there is no more movement or energy propagation.
A second way that waves are characterized is by the types of matter they are able to travel through. Electromagnetic waves (including light) can move through a vacuum. Physical waves require matter through which to propagate. Physical waves are further distinguished by the phases of matter through which they can move.  Longitudinal waves can pass through liquids and gasses while transverse waves require a material to be solid to propagate.

Below is a netlogo Java applet* designed to illustrate particle and energy motion for the three wave types listed above.   Below the model is a youtube video demonstrating the use of the model.

Demonstration video:

*The applet requires Java 5 or higher.
Java must be enabled in your browser settings.
Mac users must have Mac OS X 10.4 or higher.
Windows and Linux users may obtain the latest Java from
Sun's Java site.

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