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

It’s actually sort of the same thing that causes both of them, in a sense.

Light and paint work differently. Paint works by absorbing some colours and leaving others. So red paint absorbs everything except red light, so we see red. Blue paint absorbs everything except blue light, so we see blue.

Light works in an additive sense. We have cone cells (light sensors) for red, green, and blue light. If we see a red light, the red cones fire. If we see a green light, the green ones fire. If we see yellow light, in between red and green, both fire a bit and we see yellow. However, if we see red and green light mixed together, we see yellow as well as we trick the brain by seeing a bit of red and a bit of green.

Purple is a funny one. If we see some red light and some blue light, our brain has to make up a colour. It can’t be between red and blue because that would be green except the green cones aren’t firing. So it invents purple.

Then we get purple paint. This is a mix of red and blue, meaning it absorbs some of the red light, some of the blue light, but all of the green light. I.e. the light that reaches our eyes is effectively *not green*, but has some red and some blue. So, we see purple.