Big Spring
Observe longitudinal waves
Watch the movement of the spring which generates a longitudinal wave travelling in the same direction as the oscillating particles. The particles alternatively thicken or become thinner like an accordion. This is typical for the propagation of sound in air. If we permanently and regularly wave one end of the rope while the other end is fixed, the wave propagates to the other end, where it is reflected and returns. This is accompanied by the interference of the direct and the reflected wave and the result is a standing wave which seems to be standing in place. This is typical for the propagation of sound in air.
