We will consider a resonant cavity to be a waveguide of length with caps at both ends. As before, we must satisfy TE or TM boundary conditions on the cap surfaces, either Dirichlet in or Neumann in . In between, we expect to find harmonic standing waves instead of travelling waves.
Elementary arguments for presumed standing wave -dependence of:
For TE modes must vanish as the only permitted field component is
Given these forms and the relations already derived for e.g. a
rectangular cavity, one can easily find the formulae for the permitted
transverse fields, e.g.:
However, now is doubly determined as a function of both and and as a function of and . The only frequencies that lead to acceptable solutions are ones where the two match, where the resonant in the direction corresponds to a permitted associated with a waveguide mode.
I leave you to read about the definition of :
I strongly advise that you go over this on your own - describes the damping of energy stored in a cavity mode due to e.g. the finite conductivity of the walls or the partial transparency of the end caps to energy (as might exist in the case of a laser cavity). If you go into laser physics, you will very much need this. If not, you'll need to understand the general idea of to teach introductory physics and e.g. LRC circuits or damped driven harmonic oscillators, where it also occurs and should know it at least qualitatively for e.g. qualifiers. I added an optional problem for resonant cavities to the homework assignment in case you wanted something specific to work on while studying this.