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, the banana looks…
yellow. That’s not very surprising. Because bananas are yellow. Unless I put it under red light, then, it
looks red. Green light, gives us a green banana. But under blue light, it’s black. That is… 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: a) color vision
is way more complex, mind-boggling, and amazing than I ever imagined, and b) that to figure
out how it works, we’re gonna have to take a little bit of a journey together. But if you stick with me, I can pretty much
guarantee you you’ll never look at colors the same way ever again. [OPEN] 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, or 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. 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. Of the light we can see, violet has the shortest
wavelength, and red has the longest wavelength. A machine in a lab can read the inherent color
of light by measuring its wavelength. Bleep bloop, that’s yellow. But our eyes don’t see color the way machines
do. Take a look at this. Two boxes, one with a grey x, and one with
a green x. But what If i told you, both Xs are the exact
same color? Because they are. This doesn’t fool a machine, but it fools
us. Clearly there’s more going on here. Let’s review a little eye-natomy real quick. At the back of the eye, in the retina, are
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 the 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. Like how a camera works. But they realized if that were the case, these
would be all the colors we could see. You might notice a few missing. Like banana yellow. There’s clearly something more going on
here. It has to do with the fact that the same color
of light can be absorbed by multiple cones. This means things can get pretty weird. Let me show you what I mean. Yellow light has a wavelength of around 580
nm. 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 in which cone
a particular photon hits, comes 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 light, but
absorb blue – that’s why the banana looked black under a blue light. But under both red and green, 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 it out – and
it turns out that it does this by weighing certain colors against each other. One Austrian scientist noticed 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 vs. greenness, blueness vs. yellowness,
and dark vs. 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, now keep 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 is turned down. And vice versa. We can see this in action by looking at the
shadows cast by colored lights. If I overlap all three, where blue and red
overlap, we get a bluish red. Where green and blue overlap, greenish blue. But here, where red and green overlap, there’s
only yellow… or anti-blue. Red and green give the sensation of yellow,
the opponent of blue. This opposite color thing might seem like
an 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 aren’t computers, and our eyes
don’t work like a camera. It would be a lot easier if they did. The camera filming me now can take in amounts
of red, green, and blue light and mathematically integrate that information to give us a value
for each pixel, that 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: R v G, B v Y, K
v W. Using biology, to catch photons, and make the best guess we can. And sometimes we guess wrong. Remember those Xs? All it takes is a little bit of separation
to reveal what’s really there. So what color is anything? Turns out things are the color they are, but
they’re also not. And that’s weird. Now if you’ll excuse me, I’m going to
go rest my eyes. Stay curious.
