Purple is a mixture of colors, like white. If you mix blue light and red light, your eye will see purple, but in reality, it’s just a mix of blue and red. Why does the human eye see purple as if it was a real color, like green or yellow? To understand this, we need to take a voyage into the human eye.

At the back of the eye is an amazing tissue called the retina. The eye projects an image of the world onto the retina, where special cells receive the light and send signals to the brain. Our color vision comes from certain cells called cone cells. There are three types of cone cells. One reacts mostly to red light, another reacts mostly to green, and the third reacts mostly to blue. That’s why red, green and blue are the primary colors: The human eye can really see only these three colors.

So how do we see the color orange? If the brain receives signals from lots of red cone cells and some green cone cells, we see orange, because orange is closer to red than green. The human eye can’t tell the difference between a beam of pure orange light and the right mixture of red and green. The difference is that true orange light would not be affected by a prism, but a prism would separate a mix of red and green.