- 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 on a stretched spring fixed at one end
- In this section, we will look at a few experiments that demonstrate stationary waves in everyday life
- 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 an oscillator vibrating a length of string under tension fixed at one end:
Stationary wave on a stretched string
- As the frequency of the oscillator changes, standing waves with different numbers of minima (nodes) and maxima (antinodes) form
- 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
Using microwaves to demonstrate stationary waves
- 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 fine powder inside the air column and a loudspeaker at the open end
- 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 wave in an air column
- 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
Nodes and Antinodes
- A stationary wave is made up nodes and antinodes
- Nodes are where there is no vibration
- Antinodes are where the vibrations are at their maximum amplitude
- The nodes and antinodes do not move along the string. Nodes are fixed and antinodes only move in the vertical direction
- Between nodes, all points along the stationary wave are in phase
- The image below shows the nodes and antinodes on a snapshot of a stationary wave at a point in time
- L is the length of the string
- 1 wavelength λ is only a portion of the length of the string
A stretched string is used to demonstrate a stationary wave, as shown in the diagram.
Which row in the table correctly describes the length of L and the name of X and Y?
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.
Can't remember which is the node and which is the anti-node? Nodes occur at areas of NO Disturbance!