Purple is a weird colour. The formal name for purple is
magenta, and the weird thing about magenta is that you won't ever see it in a rainbow. And the rainbow is supposed
to be the full spectrum of colours. So why doesn't purple,
why doesn't magenta appear in the rainbow? And the answer is to do
with colour mixing. I've always had a problem with
colour mixing, because I know that you can't mix
photons together. So you can't take a blue photon
and a green photon and mix them together to get
some other photon. That just doesn't happen. And yet, you can mix paints
together in art. Color mixing is definitely
something you can do. So what's the answer? Well actually, you can't mix
colors together in physics, but you can do it in biology. It's to do with how
your eyes work. For example, if I shine red
light and green light into your eyes at the same time, if
I cross these over, you will see yellow. So when you mix red and green
together, you get yellow. And if you look at the spectrum,
yellow is in between red and green. So maybe that's the rule for
mixing colours together. You mix two colors together, you
get the colour in between on the colour spectrum. And we can test that again,
so I'll look at green and blue together. So if I mix green and blue
together, I get cyan. And cyan is in between blue
and green on the spectrum. So that's great, you mix two
colours together, you get the colour in between. But why, why is that? Well, your eyes can't
measure the wavelength of light directly. So it's not like a photon comes
in, and you know, it's 200 nanometers or whatever,
and it detects that. Instead, you have these cone
cells at the back of your eyes that are sensitive to different parts of the spectrum. So when red light comes into
your eyes, there's a set of cones that fire and tell your
brain you're looking at something red. So we'd call those
the red cones. There's another set of cones
that are more sensitive to green, so when there's green
light going into your eyes, they fire and they send a
message to your brain. And there's blue
cones, as well. So you've got red cones, green
cones, and blue cones. So what about yellow? What about when you're looking
at yellow light, like that? Well in that situation, you
don't have a yellow cone. So what do you do? Well, yellow is quite close
to red, so your red cone fires a bit. And yellow is quite close to
green as well, so your green cone fires a bit. So your brain is getting a
message from your red cone and your green cone at the same
time, and it's deciding, OK well, I must be looking at
something in between those two colours, then. And that's brilliant, because
your brain is perceiving something about the world
that it isn't able to measure directly. It isn't directly sensitive
to yellow light. It does mean that you
can be tricked. And so if I make red light and
green light go into your eyes, but no yellow light, you
will see yellow. Anyway. So, go red torch and a green
torch, and there's no yellow light, here. But when I combine them, you
will see yellow, anyway. And TVs do this all the time. So if you look up close
at TV, you'll see the individual pixels. And there are red pixels, green
pixels, and blue pixels. Those are the only colours being
produced by your TV. And yet, they can produce all
the other colors with this trick of colour mixing. So what about purple? What about magenta? Well, what should your brain do
if your red cone fires at one end of the spectrum and your
blue cone fires at the other end of the spectrum, but
your green cone doesn't fire? Does it do the same trick? Does is think I must be looking
at colour in between red and blue? When the colour between red and
blue is green, and you're definitely not looking at
something green, because your green cone isn't firing. So in that situation, your
brain invents a colour. It makes up a color, and that
colour, is magenta. And I can show you that with
my red and blue torches. So when they're combined
together there, you see magenta-- absolutely beautiful. And that's why you don't see
magenta in the spectrum. You don't see magenta in the
rainbow, because it doesn't have a wavelength. It's just the absence of
green, if you like. Just to show you the full
palette of colours that you can see on a TV screen-- so you get red and blue mixed
together makes magenta. Green and red makes yellow. Green and blue make cyan. When you mix them all together,
you get white. So when your red cone, your
green cone, and your blue cone are firing together, you
get white light.
A) Those "torches" are amazing, how do I get those?
B) I thought violet was on the spectrum, though?
That's why Barney is a purple dinosaur. Those kids used their imagination to bring him to life.
Super CRAZY incomplete without spectral violet in the discussion.
The "short wavelength" cone isn't a "blue cone". It's a cone that is most sensitive to violet, and falls off as you move away from that.
Violet light pretty much JUST stimulates this cone, with high wavelength ("red') and medium wavelength ("green") not firing.
Blue light stimulates this "short wavelength" cone, but ALSO to a degree stimulates the "medium wavelength" cone (green). So when you see blue, what is happening is that the high/medium wavelength cones are being combined and subtracted from the low wavelength input- so you are looking at "violet and green", and you sense that this is blue.
When he shines red and green light together, the red and the green are being subtracted. The brain knows that there is light, doesn't have any "low wavelength cone" input, and by looking at the difference between "high" and "low" decides that on the red/yellow/green area, it's mostly yellow.
In the purple case, you have BOTH of those things happening. The difference is, unlike the "blue" case, the green is now being "cancelled out" by the red. So the complementary cells that are there to subtract red from green are saying that the light is closer to neutral on that axis than it was when there was just blue light (and the greens were winning) or just red light (and the reds were winning). If you were to add actual green to this, the "short - high+med/2" type logic would no longer favor "short", and you'd see white- but while that isn't present, it still favors "short". So it's the same situation at that stage of processing that you would get with a spectral violet input.
You're basically spoofing the inputs to get the "this is violet" answer out of that processing. It's true that purple doesn't exist, but this is why it looks so much like violet- different inputs to get the same output.
This guy will be devastated when he hears this.
Purple is a pigment of my imagination.
His breathing is very unsettling
It's funny how we see exactly the same colour for pure yellow light or a combination of pure red and pure green light, but they're physically entirely different. Our eyes are just unable to perceive the difference.
But this is all specific to our species! For example, we all know dogs see less colours, but not only that, they also see different colours. So when you turn on your tv and your dog is watching with you, even the dog will notice that the colours are completely wrong. Colour tv's work only for human colour vision.
Some other animals see more colours than us instead of less. That means that for them, there are many colours like purple that don't correspond to any pure light source. Can you imagine that there are thousands of extra colours that we are just unable to see? What would they look like?
Purple isn't on a raimbow? What?
THEN EXPLAIN THIS
All I can focus on is him sucking air through his teeth.