Scroll down to related links and look at "Frequency spectrum -
Wikipedia"
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The energy of EM radiation depends on its frequency, and the
lowest frequency corresponds to the lowest energy. Radio waves are
at the low end of the energy and frequency spectrum.
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The so called visible spectrum. (from red [low frequency] to
violet [high frequency].
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The range of the visible spectrum is 400THz - 789THz.
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The lowest frequency is effectively zero and this would be in
the extremely low frequency (ELF) band in the sub-radio range of
the electromagnetic spectrum.