The presence of two perfectly conducting plates introduces a strong constraint on the properties of the electromagnetic field in the region of space BETWEEN THEM.

In fact, while in the OUTER REGION one can have oscillation modes of any wavelength, in the in the SPACE BETWEEN PLATES only those particular types of oscillations (modes) could exist, for which the electric field is zero on both plates. This means that there is a maximum possible value for the wavelength of electromagnetic waves in the region between the plates: the maximum wavelength  is such that one half of it equals the distance between the plates themselves.