When two progressive waves of the same amplitude, frequency, and velocity traveling in opposite directions superpose in a region of space, the resultant wave is confined to that region and can’t progress through the medium. Such a type of wave is called a stationary wave or standing wave.
Characteristics of Stationary Waves:
1. Two identical but oppositely directed progressive waves superpose to form a stationary wave.
2. Along a stationary wave, the particles at different points vibrate with different amplitudes. The points at which the amplitudes of vibration are always zero are called nodes. The points with the maximum amplitudes of vibration are called antinodes.
3. The distance between two consecutive nodes or two consecutive antinodes is λ/2 (where λ = wavelength of the stationary wave).
4. Nodes and antinodes are fixed points. They don’t change their positions with time. So, stationary waves don’t travel through the medium. They remain confined to a region.
5. All the particles in a loop, between two nodes are displaced in the same direction relative to their equilibrium positions. So, these particles are in the same phase.
6. Particles in adjacent loops are displaced in opposite directions at any instant. So, they are in opposite phases.
7. All the particles come to rest simultaneously twice in each period and they cross the equilibrium position simultaneously twice in each period.
8. At the instant when all the particles are at their equilibrium positions, the potential energy of the stationary wave becomes zero, but the kinetic energy becomes maximum.
On the other hand, in the maximum displaced positions, the kinetic energy becomes zero, while the potential energy becomes maximum. The total energy of the stationary wave is always conserved.
9. The changes in density and pressure are maximum at nodal points but zero at antinodal points.