The color purple does not exist in the real world.
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.
Light is electromagnetic radiation, 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 color thanks to three different types of color receptor cells, or cones, in our eyes. Each type of cone is sensitive to a range of colors, 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?
What we perceive as color 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 color.
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 color and as such I would suggest that it is the most intimate and personal of colors, created purely in the depths of our perceptual system. This is not true of any other color but regal, sensual purple.
Our flag is in process here is a teaser: