Stationary wave (definition)
● VERBATIM — examined word-for-wordA stationary (standing) wave is formed by the superposition of two progressive waves of the same type, amplitude, frequency and speed (hence the same wavelength) travelling along the same line in opposite directions.
What this actually means
Five conditions and every one is examined: same type, same amplitude, same frequency, same speed, opposite directions along the same line. Miss one and you lose a mark.
Same amplitude is the one people drop. If the amplitudes differ, the nodes never reach zero and what you get is a partial stationary wave, not a proper one.
In practice the second wave is almost always the reflection of the first. That is why every real demonstration in this topic (string, pipe, microwave and reflector) is a source plus a boundary.
The frequency of the stationary wave equals that of the component progressive waves. The wavelength of the stationary wave is also that of the components, which is why node spacing is λ/2 and not λ.
Do not say the wave stands still because it has no speed. Say the wave profile does not propagate, so no net energy is transported along the medium.
Omitting equal amplitude from the list of conditions, which is the mark most often lost on this definition.
Prove it — watch it be true
- Open the graphical formation view showing the two component progressive waves.
- Confirm the two components have equal amplitude, equal wavelength and opposite travel directions.
- Play the animation and watch the resultant envelope stay fixed in position while the components keep moving.