In figure 4.3 a graphic representation is given for allowed frequencies inside a laser cavity.

Figure 4.3: Allowed Longitudinal modes inside a Laser Cavity of length (L) and index of refraction (n).

In practice, the frequencies are not defined mathematically as single frequencies, but each have a width of frequencies around the possible modes, as will be explained in chapter 5.

Not all these allowed frequencies will be emitted from the laser, since there are more limiting conditions which will be explained in chapter 5.