Stationary waves
- Stationary waves, or standing waves, are produced by the superposition of two waves of the same frequency and amplitude travelling in opposite directions
- This is usually achieved by a travelling wave and its reflection. The superposition produces a wave pattern where the peaks and troughs do not move
Formation of a stationary wave
Formation of a stationary wave on a stretched spring fixed at one end
Stretched strings
- Vibrations caused by stationary waves on a stretched string produce sound
- This is how stringed instruments, such as guitars or violins, work
- This can be demonstrated by a length of string under tension fixed at one end and forced to vibrate due to an oscillator:
Standing wave experiment
Stationary wave on a stretched string kept taut by a mass and pulley system
- As the frequency of the oscillator changes, standing waves with different numbers of minima (nodes) and maxima (antinodes) form
Microwaves
- A microwave source is placed in line with a reflecting plate and a small detector between the two
- The reflector can be moved to and from the source to vary the stationary wave pattern formed
- By moving the detector, it can pick up the minima (nodes) and maxima (antinodes) of the stationary wave pattern
Stationary microwaves
Using microwaves to demonstrate stationary waves
Air Columns
- The formation of stationary waves inside an air column can be produced by sound waves
- This is how musical instruments, such as clarinets and organs, work
- This can be demonstrated by placing a loud speaker at the open end of an air column with fine powder inside
- At certain frequencies, the powder forms evenly spaced heaps along the tube, showing where there is zero disturbance as a result of the nodes of the stationary wave
Stationary waves in an air column
Stationary waves can be seen in air columns using dry power
- In order to produce a stationary wave, there must be a minima (node) at one end and a maxima (antinode) at the end with the loudspeaker
Examiner Tip
Always refer back to the experiment or scenario in an exam question e.g. the wave produced by a loudspeaker reflects at the end of a tube. This reflected wave, with the same frequency, overlaps the initial wave to create a stationary wave.