How come red & purple are on opposite, far ends of the electromagnetic spectrum, but when mixing colors together in kindergarten, purple is halfway between red & blue?

Question

Shouldn’t common sense dictate that either both be one, or both be the other?

In: 1934

Answer 1

I think there are two ways to percieve purple:

A:

1. Genuinely “purple” light enters your eyes. (Light with wavelength ~400nm)
2. This triggers both the parts of your eye that detect red, and those that detect blue.
3. You perceive this as purple.

B:

1. A mix of red and blue light enters your eyes. (Red at ~700nm and blue at ~470nm)
2. This triggers both the parts of your eye that detect red, and those that detect blue. [Exactly like scenario A]
3. You perceive this as purple. [Exactly like scenario A

You cannot tell scenarios A & B apart. Your eyes and b rain are not a perfect spectrometer that gives you objective information about the light that hits your eyes.

They are instead an approximated filter that gives a useful (but incomplete) subjective idea of the light around you.

One of the not-quite-true bits of information is that scenario A and B are the same.

—–

Similar things can happen for other colours, but in those cases, your eyes are less-wrong, because you’re basically detecting the average wavelength of the two mixed together.

Like wavelength wise, Red & Green combined feel like Yellow, and Yellow has a wavelnegth half-way between Red & Green. So perceiving the mix as the average isn’t quite *correct*, but it is still a good approximation.

It doesn’t really matter that we’re more-wrong about this with purple, but the idea that the colour wheel ‘loops around’ at purple is an illusion due to how our eyes and brains are wired.