Unless I've done my math wrong, it's roughly a doubling of frequency between the two ends of the spectrum, that makes an octave. From Wikipedia: “A typical huma

Unless I've done my math wrong, it's roughly a doubling of frequency between the two ends of the spectrum, that makes an octave. From Wikipedia: “A typical human eye will respond to wavelengths from about 380 to about 750 nanometers.[1] In terms of frequency, this corresponds to a band in the vicinity of 400–790 THz.”