The visible range of the electromagnetic spectrum covers the continuum of wavelengths between about 800 and about 350 nm (nanometers). In terms of energy, which is inversely proportional to wavelength, this is a progression from lesser to greater energy. And here there is an interesting disconnect between the way humans perceive color and the physical truth of the matter. School kids learn the mnemonic
Now the real challenge: Having observed this discrepancy, what we might call a “discontinuity” of perception, can we imagine a way to exploit it in a useful invention?
My first thought: we have two colors between violet and red which are infinitesimally different, which amounts to imperceptibly different, to the human eye, yet are dramatically different in terms of energy, which electronic devices should be able to detect. So if we oscillate a signal between those two optical frequencies we should be able to transmit data in the optical range in a way that is invisible to the human eye yet readily machine-readable. So we could have a light flashing coded information that a machine could detect but which would appear to be constantly shining to the human eye. Now, what good is that?
I dunno yet. Visual radiation has the advantage that the atmosphere is generally transparent to its propagation (which is why our eyes have evolved to see it). but so do radio and lots of other invisible waves. So the key is that we want a signal that is necessarily visible for some reason, but in which machine readable information can be transmitted invisibly. What is the application?