ELI5: If red and purple are at opposite ends of the visible spectrum, why does red seem to fade into purple just as well as it fades into orange? |TTI

Wouldn’t it make sense for red to fade into green or yellow more smoothly than purple? They are both closer to red in wavelength than purple.


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  1. This question leads down a rabbit hole. Ready?

    **Purple is not on the visible spectrum**

    ROYGBIV. No P at all. Violet =/= purple.

    Violet is not the same as purple. Let that sink in. The similar color is an illusion. Violet is actually a color we can’t really precieve (directly). Purple is a mix of red and blue pigment. Violet is the thing to the right of blue on the rainbow. Purple is a fake color – so is brown.

    We “see” violet because of harmonics. We don’t have a violet color receptor; just red blue and green. There is a sensitivity in the red cone that makes it activate a tiny bit from violet light. Thus is essentially a harmony like in music – because the wavelength is almost doubled. Notes have the same similar sound to their harmonic partners.

    Because this is similar to a red mixed with a blue (purple) our brains use the same sensation to represent them. In reality, they are as different as yellow and indigo.

    **Edit: people seem interested so here is more of the rabbit hole**

    They sky isn’t blue.

    Ever heard of Rayleigh scattering? This is the explanation often given for why the sky is blue. It states that nitrogen and oxygen (thanks /u/rrtk77) refract light to favor shorter wavelength and it’s true. But violet is shorter than blue. So why isn’t the sky violet?

    **The sky is violet**
    If you hold a colorimeter up to the sky, it will tell you that your eyes are lying to you. The sky is actually violet but our eyes don’t see violet very well (for the reasons above).

    **Edit 2: pink is also not real**

  2. Gah, some people responding here need to just not.

    Alright I’m going to spend some time breaking down how colour works for you before getting to the why. It’s needed background information, but feel free to skip over the bits you know.

    Colour as we perceive it is not something inherent to the spectrum of visible light. It’s related, but there is nothing about a photon with a 2.61whatever eV of energy that makes it inherently or objectively blue. The photon does not give a shit, it’s just a photon that carries some energy and has the related wavelength.

    The perception of colour is a product of biology. Your eyes send an electrical signal to the brain and your brain interprets that information. It turns out being able to easily distinguish different objects is useful what for not getting eaten and finding things to eat, and most materials reflect different and unique parts of the light spectrum. Thus the eye evolved to give more detailed information to the brain and the brain evolved to process that into what we perceive as colour, thus reducing your chances of getting eaten by wolves or licking a poison dart frog.
    So how does the eye work. I’m sure you’re aware that you have specialized cells in your eyes called rods and cones; rods cells are responsible for low light vision, cone cells for vision in bright light. You’ve also probably been told that rod cells are achromatic which is why you don’t see colour in darkness, but cone cells are chromatic and allows you to see colour in other times. This is accurate enough but a little overly simple. Each one of those cells, cones and rods both, contains a pigment, and like any pigment it absorbs light around a certain wavelength and reflects others, and when it absorbs light, this produces a response in the cell to the brain. They also respond most strongly at a certain wavelength and then falling off in a general bell curve type shape to the left and right of that peak.

    You can see an image of that [here]( (note, not to scale, comparative only) but to list:

    Rods activate between 400nm and about 640nm, with their peak around 510nm.

    Blue cones activate between 370nm and about 550nm , with their peak around 420nm.

    Green activate between 400nm and about 700nm with their peak around 530nm.

    Red activate between 400nm and about 700nm, with their peak around 560nm.

    This is why the light off a 420nm ish laser looks really really blue. It’s activating the blue cones in your eyes very very strongly, and everything else much weaker. However most things aren’t lasers and don’t produce/reflect monochromatic, instead they reflect multiple different parts of the spectrum. That results in more than one wavelength of light hitting your eyes at the same time. You’ll also notice there’s quite a bit of overlap between the ranges of activation for the cells in your eyes. It’s that overlap combined with mixed wavelengths triggering more than one type of cell at a time that gives rise to the rest of the colours. So while the visible spectrum looks like [this]( the diagram of colours we can percivce looks like [this](

    So now we can get into the why. That last diagram there is a C.I.E. Chromaticity chart. The X,Y coordinates describe the relative strength of activation of each of the cone cells in your eyes. Closer to the bottom right, red is most strongly activated , bottom left blue and top left green. If I point violet light at your face (so bottom left) and then start to also add in red light by increasing the intensity of the red you can follow the charting from violet and start moving into purple into a sort of light fuchsia to a dark fuschia and so on. Eventually there’s just tons of red wavelength light bombarding your eyes, in comparison to blue wavelength light and the red overwhelms the blue and you see nothing but red. This is also why a really really bright violet wavelength light appears blue, but bright purple still looks purple. Intense violet light just triggers blue cone cells strongly so our brain goes “oh that’s really blue”, but intense purple still maintains that mix of red and blue we perceive as purple

    Also contrary to what you may have heard, rods do play a role colour vision, however not all of the time. You need light that’s dim enough for rods cells to be activated, but not too dim that the cone cells stop activating. This is why colours get a bit funny around dusk/dawn or in areas with dim light from streetlamps. Rods activate quite strongly in the sort of blue-green area of the spectrum. So when the light gets dim reds start to become more dull or even close to black, but you’re more sensitive to light in that green/blue chunk spectrum and can still make out greens and blues.

    That also why things like aircraft cockpits use red lights. Rods are not saturated by the red light and remain active thus allowing for night vision when looking outside of the aircraft, but the red light stops you vision from shifting entirely to scotopic (night) vision where the pilots would no longer be able to read their instruments. It’s also good way to observe nocturnal animals who usually can’t perceive red very well and are thus still in the dark as far as they’re concerned.

  3. Because purple isn’t actually on the spectrum. It’s not a real color at all (that is, there is no single wavelength of light that is purple). The visual spectrum runs from red to deep blue (indigo), but there’s no purple on it anywhere. Purple exists all in our head, as a consequence of how our visual system works. How is that possible?

    We have three different types of cone cells in our eyes, which detect three different ranges of light: Red, green, and blue. As you would expect, red light stimulates the red cone cells, and we see red. Same with green and blue. What about wavelengths between those colors? Well let’s take yellow as an example. Yellow light stimulates the red and green cones simultaneously, and our brain sees that as yellow.

    But here’s where it gets interesting. If yellow light stimulates the red and green cones, what happens when we shine both red and green light on the same spot of our retina? As far as your cone cells are concerned, there is zero difference between the red and green cones being activated from one wavelength of light, or from multiple wavelengths of light. So when we see both red and green light from the same source, it looks exactly like yellow light to us, because of how our eyes work (as an aside, this is how we can produce so many different colors from computer monitors, TVs, and phone screens: we just use different combinations of red, green, and blue).

    So now we get to the *really* cool part. What happens when you shine both red and blue light on the same part of the retina? Your brain wants to interpret that as a single color. But it can’t use the midpoint between red and blue (like it does for red+green=yellow), because the midpoint between red and blue is green, and shining red and blue light at your retina specifically *doesn’t* activate the green cone cells.

    So your brain invents a new color: purple! It’s a color that doesn’t actually exist in nature. There is no pure purple light. There is no single wavelength that can stimulate both the red and blue cones, but not the green ones. Purple is an “imaginary” color that is all in our head, as a byproduct of how our visual system works.

  4. You have 3 color receptors in your eyes. Colors are interpreted by your brain as a blend between those receptors. Yellow is green and red being activated. Magenta(reddish purple) is blue and red. Cyan (a sea-blue) is blue and green. All the colors you can see are an interpretation of how much each of those receptors get activated.

    In the electromagnetic spectrum, the colors of light go from red-orange-yellow-green-blue-violet. Spectral violet isn’t the purple violet that you see elsewhere. It’s because the red color receptor is a little bit sensitive to the deepest blue wavelengths of light, while the green is not. Biology isn’t perfect.

    So to answer your question why does red seem to fade into purple as well as orange? That is because orange is red, with increasing of green, where as fading to purple is red with increasing amounts of blue.

    Remember, we are talking about light here, not paint colors. Paint works by absorbing all the colors, except for what you see. Paint that is yellow is absorbing the blue light, while reflecting green and red. That is also why “blue blocker” sunglasses make everything yellow.

  5. You have three types of color-sensitive cells in your eye, which are most sensitive to red, green, and blue light. If you adjust the wavelength of a light gradually from red to green, the light will gradually stimulate the red-sensitive cells less and the green-sensitive cells more. To your eye, orange or yellow light looks just like a combination of red and green light because it stimulates the same cells. Gradually changing the wavelength looks just like gradually making a red light dimmer and a green light brighter.

    But what if you start with a red light and gradually make it dimmer while turning a blue light on? That’s the fade from red into purple and from purple into blue. There’s no wavelength of light that stimulates the red and blue cells equally, so this fade isn’t equivalent to any gradual change of wavelength. But it should still look like a smooth fade to your eye because it’s still just one type of cell being stimulated more as another type is stimulated less.

  6. I see all these replies about how we can’t see purple and now looking at the purple on my shower curtain like what color are you really?? …. I feel like my whole life has been a lie.

  7. Wow! It’s amazing what one can learn clicking on a Reddit thread. I learned more about how my brain and eyes look at color than I ever did in school just now. They say we don’t retain color in our brains either… but I once bought an outfit, then awhile later found stockings the exact same color, and a year later shoes the same color, without having any of the items with me at the time to match the color. Strangely enough it was a very bright blue with purple hue to it… I wonder how I was able to do that if my brain doesn’t retain color memory. Anyways I found this thread super intetesting. Thanks!

  8. The relationship between perceived colors and wavelengths of light is not at all simple. Wavelengths are easily described in physics terms but the physiology and psychology of vision are a whole different story with some surprising facts. For example, the eye can perceive a full range of colors even if only two rather close wavelengths are present which don’t include some of the perceived colors – ie you can perceive green in a scene where only two different red wavelengths are present physically (Land Color Theory). This shows that the brain supplies a lot of what is perceived as color vision, and it doesn’t go by obvious logic.

  9. Red and Blue are opposite ends of the spectrum but we perceive Blue and Violet fading into purple and magenta as you add red to it. The spectrum is continuous and doesn’t loop back but our vision is tristimulous based meaning we seen in buckets of wavelengths that our eyes and brains determine at Red, Green, and Blue. The buckets over lap a bit and tail off so we can get a decent idea of if a single wavelength is a red-orange, orange, yellow, or a greenish yellow but we can’t exactly tell the difference between a single wavelength in-between or a mix of multiple wavelengths that give us the same perception.

    Magenta is the perception we have that is the lack of green. So Blue+Red. There is not a single wavelength that gives us magenta but it’s the color our brains tell us we are seeing when we are seeing “not green.”

  10. The simplest answer is that colors are like sounds, they are spiral in nature. Like a fractal.

    Consider the seven major notes, and as you go higher and higher in frequency the same seven notes repeat in the same order. A-G, then after G the next note is A again, and the pattern repeats.

    Same thing happens with color, red-violet, then back to red again. The pattern repeats.

    That’s just how it is. I don’t know why, I didn’t program it. But… thinking about it is a great subject for times of deep contemplation or psychedelia. ????????

  11. Pigment colors work a little differently then the color spectrum. take white and black. in the light spectrum, light is white/clear when all colors in the spectrum are visible, but the absence of all colors is black. With pigments, when all colors are present you get black, and when they are all gone you get white.

    as for Red, well Purple is a mix of red and blue, so red fades in well. and Orange is a mix of Red and Yellow, so it also fades in well.

  12. Because the color spectrum is actually like a globe. if you go too far red you’ll find America and think you’re in India, sparking a mass genocide that ultimately results in my existence.

  13. People have already answered this really well regarding harmonics, but I just wanted to add that the spectrum can also be thought of as an octave of light frequencies, where each end of the spectrum is the same but a step up in frequency.

  14. Colors to do with light and colors to do with pigment at completely different.

    For instance, an absence of all colored light is darkness (black). But an absence of all colored pigment is white. On the same token, the presence of all colored light is white, and the presence of all colored pigment makes brown.

    So yes, red and violet fades together as smoothly as red and orange because they are both secondary colors that contain the primary color, red.

    For anyone who gets their feathers ruffled over being told we “can’t see violet”, we can. We have three types of color sensitive cones: low wavelength (your reds and oranges), mid wavelength (your yellows and greens), and your high wavelength (your blues and violets). The light you can’t see is ultraviolet and infrared.

  15. This is my understanding of it. If photons produce light as they travel, then the light also travels as the photon does. This would include rotation of the photon then rotating the light as it travels. When we see something like the color spectrum laid out two-dimensionally,we can rotate the entire series and can represent the entire visual (color) spectrum as a sphere. If we take red as the polar center for this frame of reference, and input orange and purple as branches, we can also see where brown resides (as evidenced by the colors Auburn and Burgundy and Rust) and that red and purple are not on opposite sides of each other, but rather are gradients.

  16. yes I am familar with resonance/constructive interference. correct me if i am wrong here,

    so light of particular wavelength when in contact with some molecule of the cones passes over an amount of energy specific to the wavelength, resulting in a change in electron state (band theory yes?) and this change in the molecule results in excitation of the cone cells and cumulates in the activation of ganglia cells.

    for resonance, i suppose the violet wavelength gives an energy similar(or similar enough to double/half) that can trigger a change in electron state, resulting in activation of cone cells?

    I suppose I have just figured out the answer to my initial question, but I may not be accurate in this line of thinking?

  17. Purple is a mixture of blue (or violet) and red. It is red “fading into” blue. There are many colors which are not single wavelengths. In fact most colors you see are a mix of wavelengths. We can distinguish between around 100 to 200 different wavelengths, but there are hundreds of thousands of distinguishable colors which are combinations.

  18. You can get any two colors to fade into each other just by adding more light of one wavelength and subtracting more of another. It has nothing to do with the color spectrum. If you have photoshop, go into the gradient tool and start setting up gradients; you’ll see that any two colors will “blend” together smoothy visually.

  19. I’m not 100% sure, but I think it’s because it’s one of the primary colours that it’s used for. If you do red and blue you get purple so red fades into it since the colour is already there and it’s just being faded out by more blue as it develops into purple.

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