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.
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