Rami Arieli: "The Laser Adventure" Chapter 4 Section 1, page 14.1 Standing Waves
As known from the wave theory, when two waves with similar amplitudes and same frequency are moving in the same path at opposite directions, their interference creates an oscillating wave which appears as if it is fixed in space - a standing wave.
In figure 4.1 a pictorial demonstration is given for the creation of a standing wave during one complete cycle (T).
In figure 4.1 the wave is described every 1/4 cycle.
The black spot show how the disturbance of the peak of the wave advances during a complete period (T).
The solid line describes a wave moving to the left.
On the right side of figure 4.1 the superposition of the two waves is shown.
Like a standing wave in a string attached to fixed points at both sides, the fixed points of a standing wave are called Nodes.
The distance between adjacent Nodes is half the wavelength of each of the interfering waves. Thus, it is the same as half a wavelength of the standing wave they create.