Colour Mixing and the Mystery of Magenta

with Steve Mould

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Seeing the absence of green...

Why doesn’t magenta appear in the rainbow? The answer lies not in physics but in biology.

Science presenter Steve Mould demonstrates the strange phenomenon of colour mixing, in which not everything is as it seems. The cone cells within our eyes are responsible for the colours we see, but are only sensitive to Red, Green and Blue light. So how are we able to see so many colours when we can only directly detect three? And, how do our brains see the colour magenta when it doesn’t appear in the visible spectrum?

With the help of his coloured torches Steve explains the mystery of magenta and explores how everyday technology fools our brains into seeing more.


Being Human


Steve Mould
Royal Institution, London

The Royal Institution

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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.

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