Do you believe in the color purple?
Obviously you believe in purple because you can see it with your own eyes. It’s part of your everyday experience. What more proof do you need?
It’s not a silly question because purple lacks the same kind of physical existence as other colors.
A prism casts a spectrum of pure light. Where is purple?
Color is how our mind perceives different wavelengths of light. Red light, the longest light that humans can perceive, has wavelengths of about 625–740 nm. Any longer than that and the light becomes infrared, light that human eyes fail to perceive.
As the light gets shorter, it passes through orange, yellow, green, blue, and violet light. Violet light at this other extreme of the light spectrum has short wavelengths ranging from about 380–450 nm. Any shorter than that and the light becomes ultraviolet, again imperceptible to human eyes.
Standard color wheels show purple between red and violet, on the opposite side from yellowish green or orange, but what wavelength can be simultaneously longer than red (i.e. greater than 740 nm) and shorter than violet (i.e. less than 380 nm)? Pick a number less that 380 and greater than 740.
No single wavelength can cause our eyes to see the color purple. It is a non-spectral color created by our minds when red and violet wavelengths are mixed together, a quale representing the presence of two wavelengths. It is a fiction.
Pure purple light doesn’t exist, yet we perceive it like the spectral colors. Our minds are tricksy like that: simplifying reality, presenting a seamless experience to our conscious awareness, and providing no caveats to what we believe is raw sensory data. If we want to know how the world outside of our heads really works, these simplifications make our work more difficult.
What other parts of our experience have our minds constructed?
I leave with another fun color illusion. Do you see green and blue spirals in this picture? Your brain has fooled you again. Both spirals have the same color: #00FF97. The context of the spirals causes our minds to present two different qualia for the same color.
