Figure 1.9 describes, using the same time base, 3 waves marked y₁, y₂, y₃, and their superposition. In figure 1.9a, the waves are coherent, like the waves out of a laser. In figure 1.9b, the waves have the same wavelength, but are not coherent with each other.
Light from an incandescent lamp is composed of waves at many wavelengths, and each wave appears randomly with no systematic relation between its phase and that of the other wave.
Laser radiation is composed of waves at the same wavelength, which start at the same time and keep their relative phase as they advance. By adding (superposition) the wave amplitudes of the different waves, higher peaks are measured for laser radiation.

Figure 1.9: Superposition of waves