Hey, smart people.

Joe here.

What color is a banana?

The answer is it depends on how you look at it.

Here I have a red, a green, and a blue light.

Illuminated by all three, these bananas look yellow, which is not very surprising because bananas are yellow, unless I put them under red light.

Then they look red.

Green light gives us green bananas.

But under blue light, it's almost black.

That's bananas.

I started out making this video about a very simple experiment to show how different colors of light can add up to make other colors.

But in the process, I started uncovering other questions, which led to other questions.

And next thing I knew, I was at the bottom of this insane rabbit hole, trying to figure out how color vision itself actually works.

And I realized two things.

Color vision is way more complex, mind-boggling, and amazing than I ever imagined.

And to figure out how it works, we're going to have to take a little bit of a journey together.

But if you stick with me, I can pretty much guarantee you will never look at colors the same way ever again.

We think humans can differentiate between more than a million colors.

But let's start with a basic question.

What is a color?

I mean, what is red?

What is blue?

We can point at things and say, that's red.

But that didn't really answer the question.

Physics gives us another way of describing color.

Now, we really didn't understand color as a feature of light itself until a guy named Isaac Newton came along and sliced white light into a rainbow using a chunk of glass.

According to physics, a color is just a unique wavelength of electromagnetic radiation in one narrow part of the spectrum.

Now, of the light that we can see, violet has the shortest wavelength and red the longest wavelength.

A machine in a lab can read the inherent color of light by measuring its wavelength.

It's yellow.

But our eyes don't see color the way the machines do.

Take a look at this-- two boxes, one with a gray X and one with a green X.

But what if I told you both Xs are exactly the same color?

Well, this doesn't fool a machine, but it fools us.

Clearly there's more going on here.

Let's review a little eye anatomy real quick.

Now, at the back of the eye in the retina are these special photoreceptor cells called "cones" that absorb photons-- tiny units of light-- and send electrical impulses down your optic nerve.

But we definitely don't have individual receptors for all of those million colors that we can see.

We do it with just three types of cones.

Each type of cone absorbs a certain range of colors of light-- short, medium, and long.

If we had just one type of cone, a photon of this color would send the same signal to our brain as this color.

And we would just see black and white.

But by comparing the signals of three cones, whose sensitivities overlap, our visual system is able to tell individual colors apart.

Scientists used to think that when an image hits your retina, each cone-- short, medium, and long-- sends its separate color signal to the brain to put together an image, kind of like how a camera works.

But they realized if that were the case, then these would be all the colors that we could see.

And you might notice a few missing, like banana yellow.

There's clearly something more going on here.

And it has to do with the fact that the same color of light can be absorbed by multiple cones.

And this means things can get pretty weird.

Let me show you what I mean.

Yellow light has a wavelength of around 580 nanometers.

When a yellow photon enters your eye, it could be absorbed by the long-wavelength cone.

But it also falls in the range of the medium cone.

So it could also be absorbed by that one.

Which light-absorbing molecule and which cone a particular photon hits come down to probability.

As more yellow photons come in, some will be absorbed by long, some by medium.

And these two buckets are filled.

Now, instead of one color of photon, watch what happens if we send red photons and green photons into your eyes.

Based on their wavelengths, both colors could be absorbed by the long or medium cones.

So again, both buckets are filled according to some probability.

But the end result, how full our buckets are, looks just like when we absorbed only yellow photons.

Our eyes interpret both of these as yellow.

You can't really tell the difference between yellow light or equal parts red and green.

In fact, this is what's happening in your screen right now.

Light from red and green pixels are hitting your eye and creating the sensation of yellow.

And that brings us back to our banana.

Banana peels bounce green and red right but absorb blue.

That's why the banana looked black under a blue light.

But under both red and green together, a banana looks yellow.

It's bouncing both colors to your eyes.

Whenever you look at something that appears some color, you're really looking at many different colors of light bouncing off of it.

And you're completely unaware of it.

But our visual system figures all of this out.

And it turns out that it does this by weighing certain colors against each other.

One Austrian scientist noticed that certain combinations of colors just don't exist.

For instance, we can perceive a combination of blue and red as purple.

But we can't perceive a color that's simultaneously blue and yellow.

I don't mean mixing blue and yellow paint or pigment to make green, like you did in art class.

I mean try to picture a color that's simultaneously blue and yellow.

You can't.

It's unpossible.

And while we can perceive red and yellow together as orange, we can't perceive a color that's simultaneously red and green.

To our visual system, blue and yellow and red and green are opposites.

To our brains, the spectrum doesn't look like this.

It actually looks like this.

Each color or hue that we see can be described by its position on each of these three channels-- redness versus greenness, blueness versus yellowness, and dark versus light.

To see this in action, take a look at this flag.

As you look at the middle, just let your eyes relax like you're looking through it.

OK. And keep on doing that.

In a few seconds, I'm going to take it away.

Don't move your eyes.

Just keep staring, and watch what happens.

See?

You should have seen something like this.

These are called "afterimages."

Where there was green, you saw red in the afterimage.

Where there was yellow, you saw blue.

And where there was black, you saw white.

Our visual system treats these colors as opposites, where one is basically the uncolor to the other.

If the sensation of one is turned up, the other's turned down, and vice versa.

We can see this in action by looking at the shadows cast by colored lights.

Now, if I overlap all three, where blue and red overlap, we get a bluish-red.

Where a green and blue overlap, greenish-blue.

But here, where red and green overlap, there's only yellow, or antiblue.

This opposite color thing might seem like a massively overly complicated way to see color.

But this system of four elementary colors sensed in three eyeball channels is what lets us tease apart a million separate hues with just three different color-sensing cells in our eyes.

Not only that, once those signals are processed by our brains and compared with our memories, we can even judge what color something is under wildly different sources of light.

Whether it's broad bluish daylight or warm orangish sunset, it's how we know this car is still yellow.

Our brains are not computers.

And our eyes do not work like cameras.

It would be a lot easier if they did.

The camera filming me now can take in amounts of red and green and blue light and mathematically integrate that information to give us a value for each pixel we can directly put on a screen or store away.

We see color with the combination of three different cells comparing four colors in three different channels, using biology to catch photons.

And then we make the best guess we can.

Sometimes we guess wrong.

Remember those Xs?

All it takes is a little bit of separation to reveal what's really there.

Stay curious.