Standing Waves

This MATLAB animation shows forward travelling (Green), reverse travelling (Blue) and standing waves (Red).

When it comes to electromagnetic wave propagation in free space we talk only about the forward travelling waves. When an EM wave encounters a boudary of two different media, portion of the wave energy gets trasmitted while the other gets reflected. These energies are quantified by the transmission and reflection coefficients for that particular boundary. When the reflection coefficient is not equal to zero, inside the initial propagation medium we observe a forward travelling wave, reflected wave or the reverse travelling wave and the standing wave. The name 'standing' is because that wave does not seem to move in either direction, rather it stays in one location with varying amplitudes. The amplitude variation is due to the constructive and destructive addition of the forward and reserve travelling waves.

This animation shows the standing wave generation when the boundary is fully reflective. The forward travelling wave is shown in Green, the reverse travelling wave in Blue and the standing wave is shown in Red. Observe the behaviror of the standing wave and how it changes in amplitude while staying in one location. The ratio between the maximum and minimum standing wave amplitudes is known as the standing wave ratio.