Standing waves are also referred to as stationary waves.
When a wave transfers energy from one point to another it is called a progressive wave. These waves travel through a medium. For example the transverse wave in the diagram below, moves through the medium it’s in. As it travel the crests of the wave move from left to right.
Unlike progressive waves, standing waves do not appear to travel.
A standing wave does not transmit energy, instead it stores it. The crests (and troughs) of standing waves form at particular points and do not move from left to right.
The oscillations of adjacent points are in phase with each other and oscillate with a different amplitude. However, the maximum amplitude of each point remains constant.
The Formation of Standing Waves
Standing waves are formed by the superposition of two progressive waves with the same frequency travelling in opposite directions.
As well as having the same frequency the waves need to have the same speed and amplitude.
When the waves are in antiphase with each other they destructively interfere and the resultant wave has a displacement of zero.
As the two waves move they become in phase with each other. This causes constructive interference and the crests of the resultant wave have twice the amplitude of the individual waves.
The alternation between destructive and constructive interference forms a standing wave, with fixed, vertical, oscillations.
Nodes and Antinodes on a Standing Wave
Standing waves have two key features that give them the appearance of being stationary. These points are called nodes and antinodes.
Nodes – points on a standing wave with minimum displacement (this is a displacement of zero).
Nodes are the result of destructive interference between the two waves and are located at fixed points within the medium.
Antinodes – points on a standing wave with maximum displacement.
Antinodes are the result of constructive interference between the two waves and form at particular places. They occur in-between two nodes and oscillate up and down between positive and negative displacement from the rest position.
The diagram below labels the nodes (N) and antinodes (AN) on a standing wave.
The dashed lines represent the wave at different points in time. As you can see the antinodes oscillate between positive and negative displacement but the nodes do not move.
The distance between two nodes or two antinodes is half a wavelength. This means that the distance between a node and an antinode is a quarter of a wavelength.
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