Wave Rarefaction: Definition & Curves

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  • 0:00 Definition of a Wave
  • 0:47 Longitudinal Waves
  • 2:09 Rarefaction & Compression
  • 3:05 Creating Rarefactions
  • 3:48 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

This lesson will explain what a wave rarefaction is and how it forms. In order to do that, we will discuss what waves are in general and how longitudinal waves contain rarefactions.

Definition of a Wave

Waves are everywhere in nature, and understanding them is an important part of explaining the world as a whole. A wave rarefaction is a particular feature of a longitudinal wave in which the vibration is parallel to the direction of motion. To explain this better, we should first answer the question: what is a wave?

Waves are vibrations in time and space that carry energy. Waves can carry enormous amounts of energy, as in an earthquake, for example. When an earthquake hits, the very first wave that arrives is a longitudinal wave; this is called a primary wave, or P-wave. Every sound you hear is also a longitudinal wave. As we'll find out later, all longitudinal waves have rarefactions.

Longitudinal Waves

As we said at the beginning of the lesson, longitudinal waves are waves in which the vibration moves parallel to the direction the wave is progressing. That might be hard to imagine, so let's get some help from a Slinky. If you stretch a Slinky out on a table and push it along its length, the energy from your hand will transmit a pulse down the length of the Slinky from one end to the other. This is a longitudinal wave, and it looks something like this:

Conversely, a transverse wave is created when a vibration is at 90 degrees, or at a right angle, to the direction the wave is progressing. That's what happens when you move that same Slinky from side to side, sending a wave down its length. Transverse waves, which include all light waves, don't have rarefactions, or compressions either, something we'll talk about in a minute. A transverse wave looks something like this:

One example of a transverse wave is the light from the sun, which like all electromagnetic waves, is a vibration of magnetic and electric fields. That vibration is at 90 degrees to the direction the wave is moving, making it a transverse wave -- it has no compressions or rarefactions.

Rarefaction & Compression

Because a wave has rarefactions, it also has compressions. Here is an image of a longitudinal wave, with its main features of rarefactions and compressions:

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