Purple doesn't exist

I’m in the midst of preparing for my PhD defence, so I thought I’d bang out something quick and completely unrelated to the thing, for those of you who haven’t come across it before.

There is this funny kind of way in which the colour purple doesn’t really exist. It obviously does—we see purple all the time. But it doesn’t so much correspond to something, but the absence of something.

Let me explain what I mean.

¶Quick primer on colour perception

Colour, for us, essentially comes about because we have cells in the eye that are sensitive to different wavelengths of light:

1. S-cones: which are sensitive to short waves, which come to us as blue;
2. M-cones: which are sensitive to more medial waves, which come to us as green; and
3. L-cones: which are sensitive to longer waves, which come to us as red.

This is what people are talking about when they’re talking about the visible spectrum of light. The shortest wavelengths we can see come through as violet. Here, the blue cell is only weakly responding to very short light waves. Then the waves get faster, becoming bluer and bluer until we get the green cell firing and we start to see cyan. As the waves get faster still, we see more and more green as the blue cell fires less and less and the green cell fires more and more. The same process as the waves get faster, only now as the red cell starts firing we start to get yellow, through orange, and into red, which will eventually become muted as the red cell fires more weakly to the increasing length of the light waves.

On the fast side of the colour spectrum, past violet, we have ultra-violet, or UV light. On the long side of the spectrum, past red, we have infra-red or IR light. We can’t see these, of course, though other animals can.¹

¶So where’s purple

What’s interesting about this, is that there is no wavelength of light that corresponds to the colour purple. Purple is a non-spectral colour. We only see purple when something reflects blue (short) and red (long) waves into our eyes, but not green (medium) waves. This is confusing to our visual system, because it knows there is something between blue and red. But that thing is green, and it’s not getting any green signals. So, instead, is just makes up a new colour for us to see. Purple is like a not, but should be green.

This sort of raises a question about why the brain chooses purple for this. Why not some other colour? Well, the most satisfying answer, to me, becomes obvious if you look at a colour wheel. When we join the longest waves (red) to the shortest (violet), we interpolate between them as purple. This way it sits opposite to green. Seems like the brain is doing more-or-less the same thing—picking the colour furthest away from green for us. So purple is also, in a sense, a symbol for all the colours we can’t see.

¶Little sidebar about pink

I also think pink is kind of fun. Pink is another non-spectral colour, in the sense that it doesn’t appear as a result of a specific range of light waves. Instead, it only appears when all three of our colour cells are active, but the red (long wave) cells are most active. The brain gets a bit confused by this, and it shows you something that is kind of red, but also obviously not. So, where purple is a not, but should be green, pink is a kind of everything, but largely red.²

It’s kind of fun that the prettiest sunsets are basically imaginary colours.