Here’s an experiment anyone can do: Go get your Apple IR remote. The LED emits at 980nm, or about 306THz, in the near-IR spectrum. Relatively speaking, this is just outside of the visible range. Take the remote into the basement, or the darkest room in your house, in the middle of the night, with the lights off. Let your eyes adjust to the blackness.

Above: Apple IR remote photographed using a digital camera. Though the emitter is quite bright and the frequency emitted is not far past the red portion of the visible spectrum, it’s completely invisible to the eye.

Can you see the LED flash when you press a button [4]? No? Not even the tiniest amount? Try a few other IR remotes; most use an IR wavelength even closer to the visible band, around 310-320THz. You won’t be able to see them either, even though they would be blindingly, painfully bright if they were in the visible spectrum.

Above top: Frequency of an Apple IR remote emitter relative to the full visible spectrum.

Above bottom: Nyquist frequence of a 192kHz sample rate audio file relative to the full audible spectrum.

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These near-IR LEDs emit at about 20% beyond the visible frequency limit. 192kHz audio extends to 400% of the audible limit. Lest I be accused of comparing apples and oranges, auditory and visual perception drop off similarly toward the edges.