There’s no purple light


Even though we can see the color purple, there’s no such thing as purple light. Purple’s not on the electromagnetic spectrum, we don’t see it in rainbows. Here’s what’s going on, there are special neurons in our eyes that detect wavelenghts of light. Let’s say this is our retina that senses light, and these are the wavelengths of light that it can detect. Let’s say the retina only had one kind of photoreceptor cell. It’ll send a signal into the brain when it senses a wavelength of light between here and here, and with these levels of sensitivity. So, for example, light of this wavelength will send a signal this strong into the brain, and light of this wavelength will send a signal this strong into the brain. This area in here is the brain, let’s say. But this photoreceptor has a problem. If there was a beam of light with this wavelength, or a beam of light with this wavelength but with less intense light, they would send the same exact signal into the brain. Which means the brain wouldn’t be able to distinguish between wavelength or intensity. Which means there would be no reliable way to tell what wavelength it’s looking at. This wavelength just looks like this wavelength when the intensity is different. Useless! But if we had two photoreceptor cells that were offset a bit and their sensitivity to wavelength, now when we send a beam of light we make two neurons fire, we get two signals. But what this does is make each wavelength have its own unique signal pattern. The proportion between the two signals is different for every wavelength. This wavelength makes a signal like this, this wavelength makes a signal like this. And they’re different. And now even if the intensity of the light changes, the proportion between the two signals stay the same. Now we have a reliable way to distinguish wavelength, and our brain processes those signals as color. Different wavelengths make us see different colors. But there’s a quirk to processing wavelengths like this. Our eyes and brains don’t know the difference between seeing two wavelengths of light, or one wavelength of light, or five wavelengths of light. So we can shine a beam of light with this wavelength. Make a signal like this, make it see some yellow. But it would create the exact same signal as if we had sent these… two beams of light with different wavelengths but coming in at the same time. On their own, they would make different colors. But to the brain there’s no between these two signals and that one signal, so we see the same color. It’s why we can mix light like this. Our brain doesn’t know the difference between a yellow wavelength of light, or red and green wavelengths of light at the same time. Sorry, this the best my flashlights can do. They’re just really different flashlights. But it sort of works, you can see it sort of working. The human eye has three of these photoreceptors, it’s the same sort of thing. But with three, we can stimulate them like this, with wavelengths at far ends of the scale, corresponding to what would’ve made us see red and blue. But this time, it creates a signal that no single wavelength can reproduce. We can go across the whole spectrum and never see the same pattern. Because a single wavelength can’t trigger these neurons in these proportions while not triggering the middle one. But our brains still don’t know the difference between two wavelengths or one. Our brains don’t even know what a wavelength is. To the brain, this is just another signal ratio, and it makes an experience for that signal. It’s not that purple doesn’t exist. Purple exists, just as much as any other color. IN YOUR BRAIN! There’s just no single wavelength of purple light. We can only see purple by mixing other wavelengths. It doesn’t feel special, does it? Violet can be seen with a single wavelength and it’s pretty similar to purple. Who can even tell the difference? I can’t. Because while there’s no reason to think to organize color into a circle, y’know, why would you do that, wavelength is like a straight line, the color wheel is intuitive and useful. At least if you include the colors we see by mixing light from the ends of the spectrum. Isaac Newton discovered the color wheel hundreds of years (HUNDREDS OF YEARS!) before light wavelength was discovered because three neurons that fire together sort of form a cyclical pattern. These two here might be sensitive to either end of the linear scale and it takes two beams of light to make them fire together. But they don’t know that. They don’t know how many different lights are hitting them. So for us, color feels like it can go on a circle. Purple is what we perceive the combined light from the ends of the spectrum. And so it feels normal. The world often scatters multiple wavelengths of light and our eyes can’t tell the difference anyway. What I love about this idea is how often it just… doesn’t come up. [MUSIC]

100 thoughts on “There’s no purple light

  1. Finally I get this!  But I feel there is still an issue with semantics.  There is no purple light wave, but logically I could define "purple light" as any combination of light waves that register in the brain as purple.

  2. Oh my gosh… notice how the edges of the violets are crisp while the edges of the purples have that red bluey light border? Or is that just me… my eyes hurt…

  3. Food for thought: If you only had 1 type of color receptor, then you can only see 1 dimension of light. That is, your vision can only see brightness. If you had 2 color receptors, then you can see in 2 dimensions of light, which is brightness and hue. Having 3 allows us to see in 3 dimensions, which we call brightness, hue, and saturation.

    Where this really gets interesting to me is with butterflies. The species Graphium sarpedon has 15 different types of photo receptors. Which means it can see in 15 dimensions because it can create so many more combinations of light that arent possible with only 3. The mantis shrip, Order Stomatopoda, is a close second with 12 different types of photo receptors.

    For anyone who wants to experience something close to this, try a quick experiment. Take a pair of cheap polarized sunglasses (they have to be polarized), pop out one lens and rotate it 90 degrees. Youll notice that some light appears to shimmer, namely light thats reflected (e.g. off cars). This is because relfected light is polarized, but because your lenses are at 90 deg, only one eye can see it. This causes the light to exist in one eye and creates a dichotomy your brain doesnt know how to show. But by doing this youre simulating a 4D version of vision, with brightness, hue, saturation, and polarization.

  4. you made a huge mistake saying we cant see purple light.
    the color you're looking for was INDIGO.
    Nice try though. lol

  5. I'm studying physics as a grad student and the concept of wavelengths is as familiar to me as the abc. But I've never heard of this. Great video!

  6. Are there other colors we can’t see normally that could be “simulated” by sending signals to the brain?

  7. It takes a brilliant mind to explain things in a way that the subject appears easy. Congrats on your brilliance. And your animation rocks as well.

  8. You can mix blue + yellow. But you don't get green. Instead you get white, because yellow is already the same as red+green. So your basically adding red+blue+green.

  9. This is why purple is a royal color. It's the color associated with higher consciousness as it's fabricated purely in the mind.

  10. great video and all, but who the hell didn't know that already? I'm 15 yo and everyone I know pretty much already knew this. And it isn't that we are just very smart intellectuals. We are normal people, on normal schools. imo, this video is pretty much like saying "smoking isn't healthy" or "touching a stove which is on hurts" or "the USA is in America". Everyone above the age of 7 (or less) knows this.

  11. Everyone's praising this video for explaining this, but it's simply wrong. There absolutely is purple/violet/whatever-you-want-to-call-it at the bottom of the rainbow. Just look at an actual rainbow or google photographs of rainbows and you'll see it.
    The red cones in your eyes have a secondary spike in sensitivity that overlaps with the blue cone. Just google "human cones" and click images. About half of the diagrams show this secondary spike.

  12. That pretty much explain why the fuck the end of the spectrum is violet and not blue, the brain think that blue without green is mixed with red, thank's.

  13. "We don't see it in rainbows"
    Funny that we are always taught that rainbows have TWO shades of purple (indigo and violet).
    I would argue that you can see some faint indigo in a rainbow. Though it could also be argued that indigo is actually a shade of blue, I suppose. But I've never considered it such.

  14. What would those same colours we perceive look as with that genetic mutation where a very rare amount of people with two X chromosomes (women who have this, or maybe even a man) also has an extra colour receptor that captures an even more diverse range of wavelengths?

  15. You really have an explanatory GIFT!!

    I couldn't understand the color perception theory in any other manual/video etc. Only with your video this all thing clicked!

  16. You're describing Magenta, not Purple, imo. Purple is a more general term that includes magenta, violet, etc. Violet light causes red receptors in the eye to fire a little bit.

  17. what if we had 4 receptor cells? what new ability could we obtain if we modified our eyes? would we see more non-existing colors?

  18. There is no white light either. Wait, what? If "light" is a mixture of different wavelengths, then there is white light, and there is purple light. White light with green filtered out is purple, that's the name we use for that mixture. The title assumes "light" to be "monochromatic light", and that's not a reasonable assumption when you are talking about perceived colors.

  19. Also, you can't see true violet on a computer screen, which can only show wavelengths between red and blue. Violet is outside that spectrum.

  20. The claim "there is no purple light" is massively misleading. Because by the same logic there is no white light.

    There is no purple monochromatic light, sure. But most colors you see in everyday life are not monochromatic anyways. *Light* is in most cases a mix of a whole continuous range of wavelengths.

  21. Oh I wish you had lingered on violet light vs. the magenta our brains create when we see red and blue light! I'm struggling to understand how our brains detect violet light given that our highest frequency receptors are for blue light.

  22. Thanks for emphasizing that "yes, purple exists" because I so often see people calling purple and magenta "fake" colors which is… not recognizing that everything you experience is just an interpretation.

  23. @This Place You briefly mention violet looking similar to purple, but fall to explain why this single wavelength appears purplish, despite red being on the other end of the spectrum.

  24. Thank you for this video! I love color theory, and sight and anything having to do with light and photons so this fits me quite nice! I like that end line of that this idea or explanation hasn’t veen brought up on youtube very much, only to certain extents, but never this thoroughly explained. In fact I had an argument about pretty much this a few weeks ago and left the discussion after having been humiliated and slandered for supposedly not arguing well and being rude, when I always stayed on topic and held onto the general definition for colour and presented facts like these without ever trying to be rude. Sorry for the rant, but I needed to see this, thanks alot to you, the guy behind ”This place”. Have a nice day!

  25. These graphical images really help to illustrate the narration, making the information quite easy to understand. Very well done!

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