When we look up at the sky on a cloudless summer day we see a vast expanse of blue space. But is it really blue? Yes and no. It’s blue in that the cones in our eyes detect it as such, but in reality the color doesn’t exist.
Newton was the first to put forth the idea that color was simply a perception, not an actual quality of any object. He concluded that the color that we see is, in fact, different wave lengths of light that are not absorbed by the object we are seeing. Or rather the wavelengths of light that it reflects back. The human eye is better than most mammals at picking up these wavelengths. This is because our eyes have 6 to 7 million cones located in a 0.3 millimeter spot within each eye. Most of these cones are located directly in front of the eye which is why we cannot see color as well in our peripheral vision.
You can experiment with the eye’s sensitivity to color by staring at a red circle on a piece of paper for several minutes. Look away from the circle to a plain white piece of paper. You should see a green circle on the paper for a short time. This is because the red has saturated your eye so that the cones responsible for detecting other colors have lost some of their sensitivity. The medium wavelength cones, which pick up the color green, are not stimulated when looking at the color red, so while your eyes adjust to looking at the white sheet of paper, where all cones are responding equally, you are seeing the color green.
So why do we see a blue sky? The air molecules in the earth’s atmosphere scatter more blue light than other light because blue travels in shorter, smaller waves. As the sun begins to set the light is passing through more atmosphere, at least from your perspective. Because of this, more red and yellow light waves are passing through to your eyes, resulting in the orange sunset.
If the human eye did not have the cones that perceive color, we would never know of its existence. Kind of makes you wonder what else is out there that we simply don’t have the sense to experience…