The colour purple does not exist in the real world. Apparently it’s true. A rainbow of light from red to violet floods our surroundings, but there is no such thing as purple light. Purple only exists in our heads. I got fascinated by what I found in exploring this purple puzzle, so I thought I’d share it …

From school we know that light is electromagnetic radiation. Part of the same spectrum that includes radio waves, and microwaves down below red, and ultraviolet and X-rays up beyond the violet end of visible light. We perceive colour thanks to three different types of colour receptor cells, or cones, in our eyes. Each type of cone is sensitive to a range of colours but one is most excited by red light, one by green and one blue.

We can see, for example, yellow light, which sits in the spectrum between red and green, because yellow light excites both our red and green cones. Green light obviously scores a major hit with the green cones, but also to a lesser extent gets the red and blue going because of the way their ranges of sensitivity overlap. As you keep heading up you go through that vibrant blue/green called Cyan (as in printer cartridges), through the blues and off into the violet by which time the red and green cones have stopped firing altogether and all that’s left is a very weak response from the blues as they reach the end of their sensitivity and we head off into the invisible ultra-violet.

So where was purple?

Violet and purple are not the same. As we know, violet is out beyond the blues. Purple is a mixture of red and blue – a very different animal. But when you look at the spectrum midway between red and blue you find green. There’s no space for purple, and yet we can definitely “see” purple. How can this be?

A clue lies in the 2 ways we can see, or perceive, yellow. What we perceive as colour is the response of our whole visual system to different frequencies of light. As well as the physical response of the cones sending electrical signals to the brain, there is the experience that our brains turn those signals into…an experience we call colour.

Photons vibrating 450 trillion times a second bump into a receptor in our eye and we experience red – a miracle in its own right. Others vibrating around 575 trillion times a second arrive and we experience green. Others vibrating at a frequency somewhere between those two arrive and we experience yellow. These “yellow” photons excite both the red and green cones, but we experience this as yellow. So imagine instead that red and green light enter our eyes simultaneously. Just as with yellow light both our red and green cones get excited. So what’s the difference? Well, in terms of our experience, there is none. We do in fact “see” yellow. We might casually say that if you mix red and green light you “get” yellow, but that’s not strictly true. What you get is excited red and green cones and you perceive that as yellow.

Okay, so what about purple? Well, if you mix red and blue light you excite red and blue cones and you experience purple. There’s no doubting that. But unlike yellow, if you look for light whose frequency is half way between red and blue to do the same job you are slap bang in the middle of the greens, and not surprisingly what you see is green, not purple at all.

So the only way we can experience purple is by seeing red and blue simultaneously. There is no such thing as purple light. No single frequency of electromagnetic radiation can give you that purple feeling. The experience of purple is an anomaly of the way we see colour and as such I would suggest that it is the most intimate and personal of colours, created purely in the depths of our perceptual system. This is not true of any other colour but regal, sensual purple.

That also, surely, gives purple a unique place in the artist’s palette. A purple pigment is not reflecting purple light into our eyes but red and blue without green. Purple is a kind of ungreen. And that gives it a remarkable relationship to the greens. These are colours that talk to one another because they have a related vocabulary. They are both talking to our red and blue cones, but one also talks to the green and the other not.

Okay, enough talking. Enough thinking and enough writing. Time to get the paints out and play.

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