Rami Arieli: "The
Laser Adventure" Chapter 4 Section 1, page 5
Standing waves in a string
In figure 4.2 the first 5 standing waves are shown.
Figure 4.2: Longitudinal modes in an optical cavity
of length L.
These are equivalent to the longitudinal
laser modes which are the modes along the optical axis
of the laser.
L = Length of the optical cavity.
n = Index of refraction.
m = Number
of the mode, which is equal to the number of half wavelengths
inside the optical cavity.
lm = Wavelength of the
m mode inside the laser cavity.
The necessary condition for these standing waves
is a Node at each end (mirror).