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Just guessing here, but if 전 means "before", then it could be similar to "infra" in this case. So if you're right about "range" then it could be "the range (of wavelengths) before 자"--whatever 자 is. (Probably not even a Sino-Korean origin for "red" because that would make the word mean "infrared", but microwaves are even further outside of the visible spectrum than infrared.)
My guess is wrong. According to Wiktionary, it comes from 전자 meaning electron which is a loan word from Chinese, which first got it as a loan word from Japanese, which created it from Middle Chinese words meaning lightning particle.