Stationary waves

A stationary wave (also known as a standing wave) is formed from the superposition of 2 progressive waves, travelling in opposite directions in the same plane, with the same frequency, wavelength and amplitude.

No energy is transferred by a stationary wave.

Where the waves meet:

➔ In phase - constructive interference occurs so antinodes are formed, which are regions of maximum displacement. 

➔ Completely out of phase - destructive interference occurs and nodes are formed, which are regions of no displacement.

A string fixed at one end, and fixed to a driving oscillator at the other gives a good example of the formation of a stationary wave:

• A wave travelling down the string from the oscillator will be reflected at the fixed end of the string, and travel back along the string causing superposition of the two waves. Because the waves have the same wavelength, frequency and amplitude, a stationary wave is formed. (Labelled combined wave on the diagram below).

The diagram below shows multiple possible standing waves on a displacement-distance graph. The blue points indicate antinodes, while the red points indicate nodes.