The visible range corresponds to the typical energy differences between different states of an outer electron in a molecule, which also correspond to the typical energy differences between the input and output molecules of a chemical reaction.

(The near infrared range corresponds to the typical energy differences between different vibrational states of the atoms in a molecule. Such energy differences are smaller than the energy differences encountered in most chemical reactions, which involve extracting or adding atoms from/to the molecule, which obviously needs more energy than the vibration of those atoms, when they remain bound in the molecule.)

Thus it is normal and expected that the output molecules of an exothermic chemical reaction may be in an excited state from which they can decay to their ground state by emitting light exactly in the visible range.

As long as it is living, in any organism a lot of exothermic chemical reactions happen. In many cases the energy produced by those reactions is used for something useful for the organism (i.e. the excited output molecules transfer their surplus energy to other molecules), but it also may escape as emitted light, reducing the efficiency in the use of the energy produced by an exothermic chemical reaction to less than 100% (the efficiency is also reduced when the energy of the excited molecule is transferred to other molecules than those intended, which eventually results in warming the environment instead of doing useful work).

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jvanderbot 3 days ago | parent | next [-]

Hooold up.

So you're saying that there is something special about the visible spectrum? I've always wondered why most eyes we know of work in that range (modulo some leftovers from our time as aquatic creatures)

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sidewndr46 3 days ago | parent | next [-]

As other's commented it is "special" because a good portion of the radiation from the Sun is in the same range.

It's also special for a few other reasons. The most obvious one being that UV light is destructive to many forms of animal life, there isn't much utility in being able to see for example something like X-Rays. They don't occur naturally in any quantity and the mechanisms that create them (lightning) also give off visible light.

On the other end of things, lower energy photons are what we would call heat. Some animals can see it, but not humans. We can sense it just fine through other mechanisms however.

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IAmBroom 3 days ago | parent | next [-]

You're missing a big one: organic chemistry* changes often occur in the 4-7 eV range of energy, which is the visible spectrum.

* Meaning "molecules containing carbon", not "hippy chemistry done without pesticides".

	▲	jvanderbot 3 days ago \| parent [-]
		This is exactly what I mean. What a fun fact thank you.

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jvanderbot 3 days ago | parent | prev [-]

This is all well and good, but the implication one level up was that there's a fundamental link between visible light and the energy levels involved in most low energy chemistry (or something).

Of course visible light is visible because it hits our eyes (is emitted by sun and is not filtered), but the comment about valence shells is quite a bit more fundamental than that.

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Y_Y 3 days ago | parent | prev | next [-]

I invite you to consider that most of the light that earth species have had available during their evolution comes from a blackbody emitter at about 6000 kelvins (solar photosphere).

https://www.sciencedirect.com/topics/physics-and-astronomy/s...

	▲	adrian_b 3 days ago \| parent [-]
		The sensitivity of the eyes is indeed matched to the available light. However the causal dependencies are more complex than this. If the available light would have been from another range of the possible frequencies, the eyes could not have used the same kinds of photoreceptors that are used now in the eyes of all animals. For instance, if the available light would have been only infrared, then photo-chemical reactions could not have been used for detecting it, but such light could have been detected by its warming effect, like some snakes do for detecting infrared. If our star would have been much colder, with negligible visible light, then such light might have been not usable for splitting water and generating free oxygen in the atmosphere. In such a case, the planet would have remained populated only by anaerobic bacteria and viruses, like in the first few billion years of Earth's history.

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benterix 3 days ago | parent | prev [-]

Yeah, I always unconsciously assumed it's just a random slice, never thought deeper about this. Thanks, HN!

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cout 3 days ago | parent | prev [-]

This is a lot to chew on. Thank you.