You ever wonder why the small band of spectrum we can see (called visible light) is special? I mean almost all animals see in light ranges are are somewhere around here. I mean sure some can see a little farther into IR or UV but in general all animals see roughly around this same narrow band of frequencies in an otherwise infinite spectrum... So what is so special?

Its actually a quantum thing. Everything roughly around visible light and higher interacts with material by electrons jumping energy levels. However IR energy is absorbed by the bonds between molecules instead, which is why IR represents heat.

Basically visible light is right where these two modes intersect. Essentially they are the lowest energy part of the spectrum that is still high enough energy to interact primarily through changing energy levels of electrons. Essentially any more energy and it wont really get through the atmosphere as extreme UV is cut off.. any lower and it wouldnt be very effecieient cause most of the energy goes to heating the eye rather than seeing... So its right in that sweet spot.

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Most contrast in the least electromagnetic bandwidth?


It is interesting that plants evolved to not use Green light.

By being able to 'see' Red and Blue, and the relative ratios, the plant knows when it is Fall.

@SpaceLifeForm The fact that they reflect green is somewhat interesting,and has some interesting consequences.

That said its response to red and blue is not usually how most plants can tell the season (there are some exceptions). Most plants detect the season based on the length of daylight not its color.

However plants do respond to red and blue light in very interesting and different ways.. many plants respond to blue light be decreasing inter nodal distances for example.

@freemo @SpaceLifeForm I've always wondered how plants know this.

However, if I might ask a stupid question, why do (for example) trees bloom and then (in the fall) lose their leaves at different dates each year? Wouldn't that be the same year to year based on length of daylight?

I'm guessing that other environmental factors also play a role causing a relativistic shift in their "IT'S FALL! LOSE THE LEAVES!" response?

I'm not a gardener nor do I remotely have a green thumb.

@jmw @freemo

Mostly energy balance. If you have a cloudy cool fall, then leaves will turn and fall sooner because the chemical reactions slow down, and the tree is not going to waste energy. It knows that it is time to pack up for winter.

It is just starting in my area, weeks later than historical.


So I spoke of "most plants" there is no one rule... There are a few common mechanisms that can be at play for plants:

* length of day
* Temperature
* inter tree communication

There are probably others I'm not thinking of.

Each plant cares about different things. For example cannabis cares **only** and length of day. So if thats all you control its all you need to get them to flower. This is typical of plants that can grow in the tropics where it doesnt get cold.

Non-coniferous trees are a bit more complex, they use a mix of signals including intertree communication, temperature, and length of day. To they may trigger later in the season if its a warm season.

Largely the difference from season to season with trees is due to temperature.


@freemo @SpaceLifeForm Thank you for the thoughtful response. I know even less re:cannabis but it's all interesting to me. GG Evolution! GG.


The point here is its very individual to the plant but its usually some combination of daylight hours and temperature.

It is very cool though how if you want to raise plant you have to know specifical what signals it so youc an control that.


@freemo @jmw

Reminds me of greenhouse days long past. We used to trick the azaleas to manage time to market.

We would shorten the daylight hours with covers over the beds. The plants were all potted, so bed in this sense was just a large tray. Large, meaning hundreds of pots.

This was to keep them from flowering. Periodically, some would be rotated into a huge walk-in cooler. Can not recall the temp, but I would guess near 50F. Great place to cool off.

Then, as market conditions decided, they would be pulled from the cooler, and given full days of sunlight and warmth. And, in a few days, they would start their bloom of flowers. The plants thought it was spring.

@freemo @SpaceLifeForm @jmw
Yep, that's a common trick that is used widely in the Netherlands.
Loads of flowers are being grown in greenhouses in the West of the country where I'm from. So on cloudy winter nights, the sky is an orange glow from all the lights that they use to coach the plants.
Lots of research goes in to it. The Wageningen University is working out which wavelengths work for which plants, so growers can use low energy LED lights to achieve the same.

@MennoWolff @freemo @SpaceLifeForm Apologies for drudging up a 2 week old conversation; But I was reminded of it earlier today when I saw this photo.

@jmw @MennoWolff @freemo

Very interesting. Looks like a High Pressure Sodium lamp that puts out enough light at the needed frequencies to keep the leaves functioning.

I can not tell for sure, but are those birch trees?

@SpaceLifeForm @MennoWolff @freemo I can't tell myself, I would need to pop into Google lens and do some image comparison. If it's green, it's probably not my forte ๐Ÿ˜…

@jmw @SpaceLifeForm @freemo
Wow that's an awesome photo.
Yes, those are definitely birches. The color and shape of the leaves is a good hint, but the white trunk gives it away.

@MennoWolff @jmw @freemo

Thanks for confirming my eyes still work.

The other choice was Aspen, but no one with a brain would plant Aspen near a building or sidewalk.

@SpaceLifeForm @MennoWolff @freemo Aspen is the tree I was familiar with with a white trunk and similar size leaves. But it also didn't make sense to me that somebody would have planted Aspen there.

@SpaceLifeForm @jmw @freemo
Interesting. I'm from Yurop, so I'm not familiar with aspens. But it now makes sense.
We were in Canada last year and I was puzzled by the odd shape of the birches there. I guess they were aspens.
I still maintain these are birches. The thin branches and general shape tell me so.

@MennoWolff @jmw @freemo

My knowledge (may be incorrect) is that birches are more higher lattitude than aspen. Birches tend to fork/split closer to ground level than aspen. Aspen spread via root system, and I not recall any birch doing this.

Both make a mess.


HPS lights are actually quite decent at growing. They are used in growing greenhouses all the time. Their more red leaning spectrum is good for lighting closer to harvest. MH (Metal Hydride) is used when the plants are younger.

@jmw @MennoWolff


It must be a very complex interplay of feedback mechanisms.

For example, a deciduous tree certainly knows when it is fall, but they can get confused before spring. I have seen them start to bud way too early during a warm spell, but then the tree figures out that is too early. Somehow, the early buds can 'see' just enough light to know that it is too early even though the temperature is above freezing.

@freemo why aren't there solar panels that use IR or UV light? Or both with visible?

@freemo Is it only a coincidence, then, that the Sunโ€™s peak energy output is at about the same spot as the middle of our โ€œvisibleโ€ light response? I always figured these were linked.


Good catch, and no not a coincident at all.. in fact the reason the sun peaks at that wavelength and the reason our atmosphere is transparent to those wavelenths is in fact the same mechanism.

I explain it here:

@freemo I thought it is because of that is where the available light in a natural environment is highest (basically sun emission spectrum multiplied with atmospheric absorption spectrum).

@phreeza Yup that is also true... but in a weird way these are the same thing... see here for a more detailed explanation that goes into this aspect:

@freemo @drnoble I think there's a more important reason: visible light includes the peak of the sun's output, and is a range in which the atmosphere is transparent.

We see with what is available.

(whether advanced life would have evolved on a planet with shorter wavelengths arriving at the surface is perhaps an interesting question - but that isn't where we live.)


While you are right, believe it or not your talking about the same thing.

If you look at black body curves of different temperatures, which is rougly what our sun is, then the peak of the light emission at almost any temperature roughly fovers around the same place.

In fact the peak of a blackbody radiation is asymptotic... mean while it does shift the hotter it gets the less it shift.. it approaches a fix point at higher temperatures. See the attached chart for an example of this, notice the verticle line defining the peake of the blackbody curve approaches a vertical line near the higher temperatures.

This asymptopic response is due to the exact same effect im describing. That is the line represents about the point where the mode of quantum interaction is inter-bond vs free electrons. For this reason you are likely to have any star with its peak in roughly the same range.

Likewise the absorption spectrum of our sky lets the peak light of the sun through because again where these two modes interact is "special" and this a lot of gases are more transparent around this region..

In fact I didnt want to go into this but solid materials are more often transparent around visible like than other parts of the spectrum slightly above and glow it (until you get too far away)... its favorability to transparency is also important so it gets through our atmosphere.

So basically while your right these effects of peak of the sun, transmission throught he atmosphere are all important yes, they ultimately all go back to being results of this same principle.


@simon_on_energy @freemo @drnoble Another factor is that visible light travels through water. Eyes first evolved in aquatic animals, and they use wavelengths that can be seen through water.


If that were the case thenw e wouldnt be able to see red light very well as it has very minimal penetration depth in water.

So while Im sure this does play a role I'm not sure its as complete an answer as it sounds.

@simon_on_energy @drnoble


In a more scifi military sense, if a space battleship wants to use its laser cannons to strafe ground targets, the laser has to use frequencies longer than the vacuum frequencies or they will be cut off by the atmosphere.

IR is great for a weapon, since heating is hurting. But far infrared gets blocked by moisture in the air.

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