Eli5: How does the colour purple “not exist”?
In: Physics
When we see colors like red, green, or blue, it’s because those colors come from specific types of light waves. Purple is different. It doesn’t come from a single type of light wave. Instead, our eyes see both red and blue light waves at the same time, and our brain mixes them together to make the color purple. So, purple is something our brain creates by combining other colors, meaning it doesn’t exist as its own light wave like red or blue does.
Our eyes perceive different wavelengths of light as different colors, but that doesn’t mean every color we perceive has its own wavelength. There’s no single wavelength of light that you can create to shine “pure purple light”; rather, we perceive purple when our eyes sense both red and blue light.
Usually, our eyes see loads of different wavelengths and more or less average them out – pure blue and green light together are perceived as yellow, the same as pure light in between those two. But red and blue are on opposite ends of the visible spectrum, and blue + red doesn’t look like the midpoint between them, which would be some shade of green. Instead our eyes perceive red light and blue light together as something else, purple.
What’s red and blue pigments combined?
Purple, right?
But what’s the wavelength halfway between red and blue?
Green.
So how does the brain distinguish between a combination of red and blue light, and the light halfway between red and blue?
By using a different set of pathways to recognize the half-and-half light differently than the in-between light.
The result is that the brain “sees” a hybrid color that can’t be represented as one single wavelength.
Our eyes perceive and process colors weird, we have three specific cones that see color, Red Green and Blue. Colors like purple are activating the Red and Blue cones, and average it out, basically making a ‘sum’ of wavelengths. That’s why we have a wide range of colors we can see, rather than only seeing Red Green and Blue.
If we could only process the individual cones, we would have some terrible colored vision, but the brain can process them together and ‘mix’ it.
If you want a somewhat related fun fact; Brown doesn’t really exist either. It’s a combination of opposite colors, but the colors in the spectrum are organized in a way that opposites never touch, but since we have the ability to process colors together to make hues, we can see brown.
When people say purple “doesn’t exist” they are talking about pure, single-frequency light.
A single photon of light will have a specific wavelength. That wavelength will correspond to a colour. We can also have multiple photons with different wavelengths, and together they will look like a different colour.
[Take a look at this chart](https://cdn-dkepej.nitrocdn.com/xHPizjaXJNONuYnLnfsGSUCsMnIlzOEq/assets/images/optimized/rev-5c79e86/blog.frame.io/wp-content/uploads/2020/02/4a-2D-Plot-Rec-709.png). Every colour along the curved edge is a pure, single wavelength colour. Everything on the inside/along the straight line requires 2 or more “pure” colours to be mixed for you to see that colour. And as the chart shows purple is along the straight line, meaning it needs to be a mix of colours.
So purple doesn’t exist in the sense that there is no purple wavelength of light. But pretty much all colour is a mix of wavelengths, so defining “real” colour as “has a single wavelength of light” is not a useful definition.
Violet is a color of the rainbow, which is a spread out spectrum of the colors of white light. These are called “spectral colors”, and you can make them yourself here: https://academo.org/demos/wavelength-to-colour-relationship/
Purple is often used to mean a mix of red and blue light, so it’s not something you can find in the rainbow. If you think that only colors in the rainbow (spectral colors) are real colors, then purple isn’t a real color. All the purples can be seen here: https://en.m.wikipedia.org/wiki/Line_of_purples
But really, violet is basically purple as far as anyone cares in everyday conversation, so the statement isn’t really true to begin with.
It does exist. Purple to us is a mix of red and blue. What internet pedants mean by it “not existing” is that there isn’t a “pure purple” light like there is for say red.
Busy mostly it’s just people saying it to sound smart. Its like saying “flavor doesn’t exist” because you can’t come up with a chemical that only has one ingredient and tasks exactly like lasagna.
Purple does exist. There isn’t a single wavelength of light that is purple you can have a wavelength that is red, orange, yellow, green, blue… but you cannot have single wavelength that appears purple. That’s because our eyes basically see in buckets of wavelengths of Red, Green, and Blue and the red and green buckets overlap a little as do the green and blue. So while we can’t exactly tell what wavelength something is, we see something that really lights up our Red cone and slightly lights up our green it looks a bit orange. The thing is for something to appear purple it has to light up Red and Blue but those don’t overlap so there’s no one wavelength that lights both of those up. We need to see red and blue but no green. So the way purple is usually seen is either white light (pretty much ”all the wavelengths”) where green is blocked out or absorbed so that only red and blue comes through or with LEDs sometimes it will just be one (or a small group) of red wavelengths and another or blue.
The premise is false, the color purple does exist. There is a very narrow definition of “color” in which purple doesn’t meet, but that doesn’t exist.
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