As I understand it, there are these categories: radio microwave infrared light visible light ultraviolet light X-rays gamma rays It seems that microwaves are really just radio waves that happen to have a very short wavelength, with the distinction that radio & microwaves are waves that can be directly produced by an electrical circuit. Is there any reason that physicists & engineers just decided that radio radiation any higher frequency than some arbitrary level would be known as microwaves? Then there is the light radiation (infrared, visible, ultraviolet). It would seem that the frequency is too high to be directly produced by an electrical circuit, so it is not considered to be a radio wave - an just happens to correspond to the radiation that stars and planets produce. Is this why it is called "light"? Or is there some other reason? Then there are the X-rays and gamma rays. I understand that gamma rays are generated by atomic nuclei (i.e., in a spontaneous release of energy, or from a fission or fusion), but what about X-rays? It seems that X-rays are generated by ionized electrons, although maybe this is simply due to the fact that ionized electrons have a super high temperature, and therefore X-rays are just the regular thermal radiation for that temperature. I guess my question is why is there a categorical limit point between microwaves and infrared light, and also between ultraviolet light and X-rays? I can understand why gamma radiation is in a class by itself, and other than the obvious categorization of light into visible, and less or greater than frequency than visible, I don't quite understand the categorizations.