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Comment by crazygringo | Hacker News Reader

> In the same way fiber optic cables in the ground use light to carry data, Taara uses narrow, invisible light beams to transmit information through the air...

If the light is "invisible" then is it even light? I looked it up and it's infrared.

Do we consider UV and infrared to be light? Or are they UV and infrared period, in contrast to light? I mean, nobody would ever call x-rays or radio waves "light". You'd never say "it's emitting infrared light", would you?

Or do they just not want to use the word "radiation" in their marketing, because that sounds scary and cancer-causing?

I'm just curious, since "invisible light" immediately jumped out to me as a contradiction in terms.

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Yes, light is any transmission of photons, and the visible spectrum is only a tiny slice of that.

> Do we consider UV and infrared to be light? Or are they UV and infrared period, in contrast to light? I mean, nobody would ever call x-rays or radio waves "light". You'd never say "it's emitting infrared light", would you?

Yes, we do. No, it's not. Yes, they do. Yes, I would.

People really say "it's emitting x-ray light" or "emitting radio light"?

I've never in my life heard that. That sounds completely wrong to my ears. (Which is why "invisible light" sounded so wrong, too.)

You are right that people more frequently say x-ray radiation. But it's the same thing. It's literally the same thing, just at a different frequency. In the same way that we have words for ice, water, and steam, but they're still the same thing, just at different temperatures.

While "X-ray light" or "radio light" have never been used in any widespread terminology, the existence of "invisible light" has become known at the beginning of the 19th century, when the terms "infrared light" and "ultraviolet light" have been coined for it.

"Invisible light" is light that is invisible for humans, but which is visible for many animals and for many of the devices that are designed to detect visible light. Traditionally, "invisible light" was distinguished from X-rays (with higher frequency) and radiant heat (with lower frequency), because those required different kinds of emitting and detecting devices than those designed for light.

In the traditional terminology, "ultraviolet" and "infrared" are abbreviations for "ultraviolet light" and "infrared light", so ultraviolet and infrared were definitely always classified as kinds of light.

However, what was initially called "infrared light" is what is called now "near infrared". What is called now "far infrared" was traditionally called "radiant heat", which is a term that dates from before it became understood that both radiant heat and light are electromagnetic waves.

The traditional divisions between radio waves, radiant heat, light and X rays were determined by the fact that each of these 4 required different kinds of emitting and detecting devices. However the evolution of technology has extended the ranges for each kind of emitter and detector, so now the ranges of the corresponding kinds of devices are overlapping, so any precise thresholds in the frequency ranges are only conventional.

The usual distinction between "light" and "non-light" is based on the underlying mechanism by which photons are being produced. Photons produced by electrons jumping between energy shells are what we call "light".

By that definition, UV and infrared are "light". Light that's outside of the visible range for our eyes, but still potentially visible to other camera sensors (and some other animals).

X-rays are not "light" because they're emitted by the atomic nucleus itself, rather than by electrons changing orbitals.

Radio waves are not "light" because they're created by macroscopic-scale movement of electrons back and forth in free space.

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