Mark and two Cats

EM Spectrum Questions from a lay-person: What are the upper and lower limits of the spectrum? I mean the uppermost- and lowermost-possible wavelengths. Also, what defines the endpoints of the spectrum? There must be some law/relationship between wavelength and energy that constrains the EM spectrum to upper and lower bounds. Diagrammes of the spectrum show it as a continuum; are gamma rays really the same as radio waves, just shorter wavelength/higher energy photons? As I said, I am just a lay-person trying to learn. Please try to keep your answer simple (if possible). TY!