Question #f4537

A graph like the following represents the response of the human eye to various colors. Three types of cells in the retina respond to red, blue, and green light.

The question has two parts:
1. Why does the red receptor respond to yellow light?
2. Why are the red and green responses so similar while the blue sensitivity is at a very different wavelength?

The answer to the first question gets into the complicated science of how color is perceived and how the brain interprets mixtures of these colors. Looking at the diagram, consider light coming into the eye with a 650 nm wavelength. Only the red sensors will see it. We can clearly say that this light contains only red. Next, consider light coming in at a wavelength of 550 nm. Both the red and green receptors will respond to this light about the same. The brain identifies equal brightness of red and green as yellow. Move over to 500 nm and the brain sees some mixture of red, green, and blue. The brain identifies this as a blue-green.

The second question is a little more about your assumptions of how color should work. When you first learned your colors there were red, orange, yellow, green, blue, and violet pictures on the board in your nursery school. Or, perhaps you had a set of blocks, rings, or carpet squares. You arranged them into patterns. You drew rainbows with your crayons. You counted them "1, 2, 3, 4, 5, 6, 7." Blue was next to green; a difference of one. Moving one more space on this number line got you to yellow.

But human vision does not respond to colored light in such a simple way. The wavelength of light which we call blue differs from green by 75 nm. Green and yellow differ by 25. The simple order you first learned in school is not really so simple when study it carefully.