The phenomenon you are describing is known as interference, specifically in the context of a system with two parallel plates. Interference occurs when two or more waves interact with each other, resulting in a pattern of constructive and destructive interference.
In the case of two parallel plates, certain wavelengths of waves will fit between the plates and exhibit constructive interference, while others will not. This behavior is primarily due to the concept of resonance and the requirement for the waves to satisfy specific conditions.
When waves are incident between the parallel plates, they can undergo reflections at the boundaries. The reflections from the top and bottom plates can interfere with each other. Constructive interference occurs when the reflected waves align in phase, meaning the crests of the waves coincide with each other, resulting in amplification. Destructive interference occurs when the reflected waves are out of phase, meaning the crests and troughs of the waves cancel each other out, resulting in attenuation.
For constructive interference to occur, the condition for resonance must be met. Resonance happens when the wavelength of the incident wave is such that an integral number of half-wavelengths fit between the plates. This ensures that the reflected waves are in phase and reinforce each other.
Wavelengths that fulfill the resonance condition will be able to fit between the plates and exhibit constructive interference, resulting in a stronger wave. Other wavelengths that do not satisfy the resonance condition will undergo destructive interference and be attenuated.
It's important to note that this phenomenon is specific to the geometry and conditions of the parallel plate system. Different systems and setups can exhibit different interference patterns and resonance conditions based on their geometry and boundary conditions.
In summary, only certain wavelengths will fit between two parallel plates in air and exhibit constructive interference due to the resonance condition. These wavelengths satisfy the requirement for an integral number of half-wavelengths to fit between the plates, resulting in reinforced waves. Other wavelengths will not satisfy the resonance condition and experience destructive interference, leading to attenuation.