Amazing article! It seems incredibly to weird to hear about transitions causing photons at 21cm wavelength; I guess I'm only used to seeing (no pun intended) much shorter wavelengths at hundreds of nanometers.
(Yeah, I know that it's a really low-energy transition, and I know about the relationship between energy and wavelength. But the net result I still find highly counter-intuitive.)
Then it will be even weirder during an MRI: The protons in your body produce a wavelength that can be of order 1-10 meters.
I mean, I understand how and why, but it feels odd.
But the wavelength of sound it makes at 20Hz is approximately 17 meter.
Wavelength is merely a human conceptualization. If we reconceptialize it as peak-to-peak interval it suddenly stops being length and becomes a time instead
From that standpoint I can confidently say that wavelength is meaningful for the sound example, but not so much for light. Someone more knowledgeable than me would probably offer better insight.