IB Physics Sub-topic C4 Notes
Standing waves
Now that you understand the properties and phenomena of waves, it is important to learn about a second type of wave: the standing wave.
Standing waves are waves whose particles travel in a constant spatial pattern whilst their amplitude continually changes.
This may be difficult to visualize, so let’s compare this to a travelling wave.
- In a travelling wave, the wave travels through the particles. Thus, every particle goes through every position in the wave, meaning all particles have the same maximum amplitude.
- In a standing wave, the particle forms the wave. Thus, every particle only goes through the up and down position of the wave in one location, meaning all particles have different maximum amplitudes.
This has two knock-on effects:
- In a travelling wave, energy will be propagated by the wave, but in a standing wave energy will not be propagated by the wave.
- In a travelling wave, all particles will have different phases, whereas in a standing wave all particles will have the same phase.
The reason it comes at the end of this topic is because you need to know about reflection before covering standing waves.
Standing wave are formed when two waves of the same amplitude and frequency move in opposite directions. This typically happens when an emitted wave reflects and meets itself again.
A common scenario where this occurs is during plucking of strings in an instrument. Because the strings are fixed at both ends, they vibrate as a wave in a cyclical pattern with a particular frequency. This means they exhibit simple harmonic motion.
Harmonics
A harmonic is simply a frequency at which simple harmonic motion is exhibited. Since musical instruments possess many notes, it means standing waves possess many harmonics.
Each harmonic has a specific spatial pattern with some particles of the wave being at rest, called nodes (N), and other particles moving in a pattern. The particles moving in a pattern with the highest amplitude are termed antinodes (A).