For whatever reason, evolution decided those wavelengths should be overlapping. For example, M cones are most sensitive to 535 nm light, while L cones are most sensitive to 560 nm light. But M cones are still stimulated quite a lot by 560 nm light—around 80% of maximum.
The reason is simple: genes coding the long wave opsins (light-sensitive proteins) in these cones have diverged from copies of the same original gene. The evolution of this is very interesting.
Mammals in general have only two types of cones: presumably they lost full color vision in the age of dinosaurs since they were primarily small nocturnal animals or lived in habitats with very limited light (subterranean, piles of leaves, etc.) Primates are the notable exception, and have evolved the third type of cone, enabling trichromatic color vision, as a result of their fruitarian specialization and co-evolution with the tropical fruit trees (same as birds, actually).
So, what's interesting is that New World and Old World primates evolved this cone independently. In Old World primates the third cone resulted from a gene duplication event on the X chromosome, giving rise to two distinct (but pretty similar) opsin genes, with sensitivity peaks at very close wavelengths. As a note, because these genes sit on the X chromosome, colorblindness (defects in one or both of these genes) is much more likely to happen in males.
New World primates have a single polymorphic opsin gene on the X chromosome, with different alleles coding for different sensitivities. So, only some (heterozygous) females in these species typically have full trichromatic vision, while males and the unlucky homozygous females remain dichromatic.
This is only tangentially related, but I have always wondered why chlorophyll absorbs blue and red, but reflects green--green being sunlight's brightest component.
It's almost as if there was some evolutionary pressure towards being very visible in sunlight which is more important than evolving ways to collect as much sun energy as possible. When I guess at this I end up with something along the lines of reflected green being used as a signal to a neighboring plant: "I'm already here, grow in some other direction instead." There is some evidence that plants do this (https://en.wikipedia.org/wiki/Crown_shyness, https://onlinelibrary.wiley.com/doi/10.1111/1365-3040.ep1160...) but it's not clear that the need to do so is so strong that it would overshadow the drive to collect as much energy as possible.
Or perhaps there's something to do with the physics of absorbing light to drive a chemical reaction that makes it better to absorb at red and blue while passing on green (450nm and 680nm are not harmonics--so if this is the case it's more complex than which sorts of standing waves would fit in some chemical gap or other).
Chlorophyll a, which is the pigment that actually uses solar energy to split water, absorbs red light and violet light. Thus its color is blue-green, as it can be seen in some lichens that have only symbiotic cyanobacteria.
This is most likely a historical accident, with no special meaning.
Most algae and plants have auxiliary pigments, which absorb other parts of the solar spectrum and then transfer the energy to chlorophyll a.
The land plants and the green algae use mostly chlorophyll b as auxiliary pigment, which absorbs light in a blue band adjacent to the violet band of chlorophyll a, and in a red band that is distinct and adjacent to the red band of chlorophyll a.
Thus the addition of chlorophyll b increases considerably the amount of captured energy.
The algae that are dominant in oceans, e.g. diatoms and brown algae, have more auxiliary pigments, so that many are dark brown, even close to black.
Unlike for marine algae, for land plants, capturing more solar energy is not desirable, because they already have difficulties in avoiding overheating and excessive loss of water. So the pigments used by them are good enough for their needs.
> something to do with the physics of absorbing light to drive a chemical reaction
Exactly that. Blue does two steps of the process, while red does only one. There's a cost for synthesizing all that machinery, so absorbing green would just be not worth it.
> 450nm and 680nm are not harmonics
In fact they're in 3:2 ratio with 1% margin. But they don't have to be. Take a look at fluorescence: it converts one wavelength to another, and they don't have to be multiples of each other. Once photon gets absorbed onto a chemical, the electronic structure of the molecule decides what will happen to it.
It could also be to prevent overstimulation; "maximize energy" is not really the goal. A lot of plants can die from too much Sun unless their other inputs are just right (plenty of water, etc.).
There was a proposed theory on this the spread of absorption created more stability in the power generation of plants over different conditions. This was supposed to be a more important factor than being able to absorb the peak and highest energy.
TLDR: Plants are running an energy-harvesting system that can only respond so quickly to changes in light input. Making use of green would cause variance to be large enough that the gains would not offset the losses. So, avoid green and have lower variance --> higher energy capture on average.
Also remember that these are random processes with selection pressure keeping those who survive to reproduce. Assigning a will to such processes makes them and the results harder to understand- imho.
Theres probably something more efficient at converting light into simple sugars.
There's also a chance that the primary photosynthesiizers on each happened to be purple for a while (purple earth) and the ancestors of plants absorbed red/blue and ignored green because they were getting leftovers. Also, even now, iirc the limiting step in oxygenic photosynthesis is by far rubisco's incorporation of CO2, so there's no immediately obvious fitness function that would be optimized by just increasing the efficiency of light harvesting.
This is a good biological explanation. The physical explanation is, if the sensitivities didn't overlap, our spectral sensitivity would not be continuous. There would be valleys of zero sensitivity between the cones, and a continuous wavelength sweep would result in us seeing black bands between colors.
Gray bands, or more realistically just desaturated bands. There'd still be sensitivity to light through rods (black and white), and even if the peaks of wavelength sensitivity were highly separated there would still be some cone response to wavelengths that didn't stimulate them strongly.
I'm pretty sure that line of the article didn't mean to imply that we don't know, or aren't sure, only that it goes beyond the scope of the article and isn't directly relevant to the topic at hand.
To me it looked like the circle outline had a shimmering aura, it felt very magical. This was a incredibly delightful experience so I just want to say thanks for posting it.
When the circle was around the halfway point of shrinking the color looked the most vivid for me, so be sure to wait the whole duration.
At the end, the green circle had a very intense grainy, hypersaturated quality for me. Not like the shimmering aura that shrinks with the red circle, but still there was something magical going on.
I looked away, and then back to the screen, and the effect was gone. It was only in my head. Wow.
The one with a red circle on a brown background had a really interesting effect; before the circle starts to shrink, the background becomes the same colour as the circle, then moving your eyes made a bright red or green circle appear, each on their opposite side.
I got that, as well as a seemingly random set of changes in apparent brightness. The Magenta one seemed to produce patches of bands in my vision as well, think like blown up versions of edge detection kernels.
To me, it seemed to be a visual equivalent of that auditory trick where a note seems to descend or ascend in pitch indefinitely. The outer aura of color seemed to be shrinking constantly.
The aura was a very brilliant green, which reminded me of an hallucinogenic experience I had. I was sat with friends in the city centre, my brain altered by chemical substances, and I spotted a guy 100m away with the most brilliant and beautiful green shoes. It was an amazing sight, standing out from everything else.
Sober, I later realised that they were normal green shoes, but in my no-brain-filter state, I was able to appreciate that we have many more green cones in our retina than red or blue, and normally the mix dial for green in our brain is kept pretty low not to overpower the other colours. The animation once again raised this dial to show how powerful is our raw perception of that colour.
(The evolutionary reason is that we spent a lot of time in vegetation or on trees, and it's very useful to be able to distinguish things and perceive small movements in a sea of green.)
Yes, this reminded me of a similar experience camping with friends where we could only describe the foliage as “ultragreen”. An incredibly vivid blue-green tone that suffused the whole island where we were staying. Been looking for explanations since
As the circle was about to disappear, the blue-green was super saturated for me and persisted for a good minute if I kept locking onto the white dot. Another thing you can do, is at night time (lights off) look at the red circle like before, and when the timer runs out close your eyes and keep it dark. You will see blue-green glow very strongly.
A bit unrelated but I found this interesting: water is transparent only within a very narrow band of the electromagnetic spectrum, so living organisms evolved sensitivity to that band, and that's what we now call "visible light".
I like to joke that while nitrogen gas is the most common thing around us, we are blind to it. Of course, that's a feature, since it allows us to perceive everything else further away, instead of stumbling through a perpetual fog.
This location-dependent tradeoff is something to think about when it comes to "false color" images in astronomy. If some aliens described Earth as "a boring uniform nitrogen-colored ball", we'd probably be a little offended at their ophthalmo-centrism, and tell them that the fault lies in their eyes, not in our planet.
visible light is also the last octave before you hit ionizing radiation. it’s very energetic. good for harnessing in chemical processes. not so energetic that the electrons leave the party.
At least he gave credit to HN, so the diaspora could find the source. The article is interesting. I think more needs to be said about how our eyes perceive color w.r.t. led lighting.
The sun is very close to a black body radiator, so all wavelength. The atmosphere and water filters a lot.
It is actually quite strange that plants are green -- that's the wavelength the atmosphere lets through particularly well, so would be particular good to be absorbed instead of reflected, for energy production. It seems nature hasn't come up with a good, cheap way to move the absorption into that wavelength.
> If you refused to look at the animation, it’s just a bluish-green background with a red circle on top that slowly shrinks down to nothing. That’s all. But as it shrinks, you should hallucinate a very intense blue-green color around the rim.
I do not believe I have any kind or amount of colorblindness, so imagine my surprise when extremely confused I pulled the image into MS Paint, used the Color Picker tool, and found that indeed, the background has quite a bit of blue in it.
Anyhow, I cannot reproduce the illusion cited. For me the circle just blurs out and I start seeing orange.
I did see the illusion but I just did a double-take. That image looks just straight green to me. I suppose I could imagine it being greener somehow, but blue!?
I have a slight deuteranomaly. I did see the illusion. Pretty!
> I pulled the image into MS Paint, used the Color Picker tool
The RGB values used are also indicated in the filename.
What you are seeing (in the static image) is normal. Have you noticed that (0, 255, 0) looks way brighter than (0, 0, 255) regardless of your monitor calibration? For the same reason, non-red images can have quite a bit of blue in them while still subjectively registering as "green".
Yeah, I don’t think I have any color blindness but that looked super green to me. I think I am fine at distinguishing two colors, but i am not the best at realizing the component colors I am seeing.
Just in case... Maybe you didn't wait long enough? I saw nothing so I came to the comments expecting a lot of others to day the animation is broken. However, it turns out it's a lot slower/longer than I thought. I saw the little bar on the left shrink to nothing and thought that was it and exited, but that's just the start. The full animation is the red circle shrinking all the way until it disappears. There is no illusion until the circle shrinks quite a bit.
If you make the outer colour yellow using the custom colour option, and the inner circle red, do you see a an aurora-green halo? Or if you make the outer circle yellow and the inner circle green, do you see a red halo?
The background turns green (???) eventually, kind of like as if ink started to spread across it.
Or you meant full yellow (255r, 255g, 0b) and full red (255r, 0g, 0b)?
> Or if you make the outer circle yellow and the inner circle green, do you see a red halo?
I used the controls this time and made the background full yellow (255r, 255g, 0b) and the inner circle full green (0r, 255g, 0b). Also adjusted the countdown speed, I realized I wasn't patient enough to wait out the 60s before ever (but that also it didn't need to be so long).
During countdown the entire image turned green. Whenever my eyes would move a bit, I'd see either a 3D shadow depth effect or a yellow aura around the circle. When the circle started getting smaller I just saw the yellow aura. Whenever I'd drastically move my eyes, the entire background would revert to yellow, but would quickly go back to seeing green.
I don't really see them being unusually saturated though, but maybe I just don't have a good grasp on what to expect. Maxed out R/G/B or C/M/Y all strike me as super saturated from the get-go.
I believe they are simply describing the color components, just like you don’t “see red” in a soft orange color.
Did you wait for the black bar to finish, and the circle to start shrinking? Takes a very long time. The effect happens at the edges and disappears if you remove your focus from the center dot.
I have mild achromatopsia and can see the effect in all color variants I tried.
100% accuracy, 25/25 correct. That said, some of them were extremely difficult, way more than others. Not sure if that's intentional, so I might still have color vision deficiency per se.
I also did the Farnsworth-Munsell 100 hue discrimination test, got a score of 28 on that - ideal being 0, and above 4 meaning something is amiss. So I don't really know what to make of this lol
Many years ago [1] I was led to a marvelous site explaining many subtleties of human color perception, including the three cone primaries. The author called this pure-M-cone response "psychedelic aquamarine"; the page is offline but the archive has a capture [2]. I haven't seen this color referred to by that name elsewhere, but I think it's a good name.
I think I can see psychedelic aquamarine and the other cone primaries by closing my eyes and rubbing my eyelids while pressing in gently but firmly.
I was at a physics conference once and the RGB cable for the presentations was a bit faulty and one of the color channels wouldn’t work, and this presenter’s slides were all tinted cyan. After a bit of jiggling it finally worked and the slides went back to normal, but everyone heard an old professor on the first row go “on no, now they’re purple!”
Painters have been aware of this distinction for years. I encourage interested readers to get a good artist's book on color or just head for your local art store and explore the differences between pthalo and viridian greens (or any of many other surprisingly different tonal clashes).
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The reason is simple: genes coding the long wave opsins (light-sensitive proteins) in these cones have diverged from copies of the same original gene. The evolution of this is very interesting.
Mammals in general have only two types of cones: presumably they lost full color vision in the age of dinosaurs since they were primarily small nocturnal animals or lived in habitats with very limited light (subterranean, piles of leaves, etc.) Primates are the notable exception, and have evolved the third type of cone, enabling trichromatic color vision, as a result of their fruitarian specialization and co-evolution with the tropical fruit trees (same as birds, actually).
So, what's interesting is that New World and Old World primates evolved this cone independently. In Old World primates the third cone resulted from a gene duplication event on the X chromosome, giving rise to two distinct (but pretty similar) opsin genes, with sensitivity peaks at very close wavelengths. As a note, because these genes sit on the X chromosome, colorblindness (defects in one or both of these genes) is much more likely to happen in males.
New World primates have a single polymorphic opsin gene on the X chromosome, with different alleles coding for different sensitivities. So, only some (heterozygous) females in these species typically have full trichromatic vision, while males and the unlucky homozygous females remain dichromatic.
Decent wikipedia article on the subject: https://en.wikipedia.org/wiki/Evolution_of_color_vision_in_p...
Types of opsins in vertebrates: https://en.wikipedia.org/wiki/Vertebrate_visual_opsin
It's almost as if there was some evolutionary pressure towards being very visible in sunlight which is more important than evolving ways to collect as much sun energy as possible. When I guess at this I end up with something along the lines of reflected green being used as a signal to a neighboring plant: "I'm already here, grow in some other direction instead." There is some evidence that plants do this (https://en.wikipedia.org/wiki/Crown_shyness, https://onlinelibrary.wiley.com/doi/10.1111/1365-3040.ep1160...) but it's not clear that the need to do so is so strong that it would overshadow the drive to collect as much energy as possible.
Or perhaps there's something to do with the physics of absorbing light to drive a chemical reaction that makes it better to absorb at red and blue while passing on green (450nm and 680nm are not harmonics--so if this is the case it's more complex than which sorts of standing waves would fit in some chemical gap or other).
This is most likely a historical accident, with no special meaning.
Most algae and plants have auxiliary pigments, which absorb other parts of the solar spectrum and then transfer the energy to chlorophyll a.
The land plants and the green algae use mostly chlorophyll b as auxiliary pigment, which absorbs light in a blue band adjacent to the violet band of chlorophyll a, and in a red band that is distinct and adjacent to the red band of chlorophyll a.
Thus the addition of chlorophyll b increases considerably the amount of captured energy.
The algae that are dominant in oceans, e.g. diatoms and brown algae, have more auxiliary pigments, so that many are dark brown, even close to black.
Unlike for marine algae, for land plants, capturing more solar energy is not desirable, because they already have difficulties in avoiding overheating and excessive loss of water. So the pigments used by them are good enough for their needs.
It actually peaks between magenta and blue: https://sunwindsolar.com/blog/solar-radiation-spectrum/
Green is only bright to us because of our cone sensitivities overlapping.
Exactly that. Blue does two steps of the process, while red does only one. There's a cost for synthesizing all that machinery, so absorbing green would just be not worth it.
> 450nm and 680nm are not harmonics
In fact they're in 3:2 ratio with 1% margin. But they don't have to be. Take a look at fluorescence: it converts one wavelength to another, and they don't have to be multiples of each other. Once photon gets absorbed onto a chemical, the electronic structure of the molecule decides what will happen to it.
https://www.science.org/doi/10.1126/science.aba6630
TLDR: Plants are running an energy-harvesting system that can only respond so quickly to changes in light input. Making use of green would cause variance to be large enough that the gains would not offset the losses. So, avoid green and have lower variance --> higher energy capture on average.
Also remember that these are random processes with selection pressure keeping those who survive to reproduce. Assigning a will to such processes makes them and the results harder to understand- imho.
Theres probably something more efficient at converting light into simple sugars.
This sounds like https://en.wikipedia.org/wiki/Retinal and https://en.wikipedia.org/wiki/Purple_Earth_hypothesis. Going through history, there have been times where the Earth has had oxygen spikes https://en.wikipedia.org/wiki/Geological_history_of_oxygen (Examples https://en.wikipedia.org/wiki/Great_Oxidation_Event or https://en.wikipedia.org/wiki/Neoproterozoic_oxygenation_eve...) Cool image showing how this process is unstable: https://en.wikipedia.org/wiki/Great_Oxidation_Event#/media/F...
You might be interested in the different photosynthesis cycles: https://en.wikipedia.org/wiki/C3_carbon_fixation https://en.wikipedia.org/wiki/C4_carbon_fixation https://en.wikipedia.org/wiki/Crassulacean_acid_metabolism https://en.wikipedia.org/wiki/Alarm_photosynthesis - this one was only discovered in 2016!
Research into these may have profound impact on climate change.
Deleted Comment
Just kidding of course, it is an interesting question.
Wow that's wild how heterozygousity can be that helpful. Makes you wonder if there are other genes like that.
https://jov.arvojournals.org/article.aspx?articleid=2191517
When the circle was around the halfway point of shrinking the color looked the most vivid for me, so be sure to wait the whole duration.
I looked away, and then back to the screen, and the effect was gone. It was only in my head. Wow.
A super fun, delightful experience indeed.
Edit : Apologies, I just now see the other ones.
Sober, I later realised that they were normal green shoes, but in my no-brain-filter state, I was able to appreciate that we have many more green cones in our retina than red or blue, and normally the mix dial for green in our brain is kept pretty low not to overpower the other colours. The animation once again raised this dial to show how powerful is our raw perception of that colour.
(The evolutionary reason is that we spent a lot of time in vegetation or on trees, and it's very useful to be able to distinguish things and perceive small movements in a sea of green.)
http://hyperphysics.phy-astr.gsu.edu/hbase/Chemical/imgche/w...
This location-dependent tradeoff is something to think about when it comes to "false color" images in astronomy. If some aliens described Earth as "a boring uniform nitrogen-colored ball", we'd probably be a little offended at their ophthalmo-centrism, and tell them that the fault lies in their eyes, not in our planet.
It’s interesting (kinda optimal) that different cones explore near both edges.
But I guess it could be both.
It is actually quite strange that plants are green -- that's the wavelength the atmosphere lets through particularly well, so would be particular good to be absorbed instead of reflected, for energy production. It seems nature hasn't come up with a good, cheap way to move the absorption into that wavelength.
I do not believe I have any kind or amount of colorblindness, so imagine my surprise when extremely confused I pulled the image into MS Paint, used the Color Picker tool, and found that indeed, the background has quite a bit of blue in it.
Anyhow, I cannot reproduce the illusion cited. For me the circle just blurs out and I start seeing orange.
I have a slight deuteranomaly. I did see the illusion. Pretty!
The RGB values used are also indicated in the filename.
What you are seeing (in the static image) is normal. Have you noticed that (0, 255, 0) looks way brighter than (0, 0, 255) regardless of your monitor calibration? For the same reason, non-red images can have quite a bit of blue in them while still subjectively registering as "green".
https://en.wikipedia.org/wiki/Luma_(video)
You mean this, right? https://dynomight.net/img/colors/generate.html?inside=ff0000...
The background turns green (???) eventually, kind of like as if ink started to spread across it.
Or you meant full yellow (255r, 255g, 0b) and full red (255r, 0g, 0b)?
> Or if you make the outer circle yellow and the inner circle green, do you see a red halo?
I used the controls this time and made the background full yellow (255r, 255g, 0b) and the inner circle full green (0r, 255g, 0b). Also adjusted the countdown speed, I realized I wasn't patient enough to wait out the 60s before ever (but that also it didn't need to be so long).
During countdown the entire image turned green. Whenever my eyes would move a bit, I'd see either a 3D shadow depth effect or a yellow aura around the circle. When the circle started getting smaller I just saw the yellow aura. Whenever I'd drastically move my eyes, the entire background would revert to yellow, but would quickly go back to seeing green.
I don't really see them being unusually saturated though, but maybe I just don't have a good grasp on what to expect. Maxed out R/G/B or C/M/Y all strike me as super saturated from the get-go.
Did you wait for the black bar to finish, and the circle to start shrinking? Takes a very long time. The effect happens at the edges and disappears if you remove your focus from the center dot.
I have mild achromatopsia and can see the effect in all color variants I tried.
https://www.colorblindnesstest.org/cambridge-color-test/
I also did the Farnsworth-Munsell 100 hue discrimination test, got a score of 28 on that - ideal being 0, and above 4 meaning something is amiss. So I don't really know what to make of this lol
I think I can see psychedelic aquamarine and the other cone primaries by closing my eyes and rubbing my eyelids while pressing in gently but firmly.
[1] https://news.ycombinator.com/item?id=813656
[2] https://web.archive.org/web/20160306132951/https://casa.colo...