Hold on to your hats, kids, because what you’re looking at is not at all what it seems. In the image above, the girl appears to have one turquoise eye and one grey eye. But if you were to examine the RGB values for each eye, you’d find that they are exactly the same. According to Scientific American, this is because of opponent process:
The girl’s right eye only looks the same as the turquoise hair clip because of the reddish context. Part of the process of seeing color is that three different kinds of photoreceptors in the eye are tuned to three overlapping families of color: red, green and blue (which are activated by visible light of long, medium and short wavelengths). These signals are then instantaneously compared with signals from nearby regions in the same scene. As the signals are passed along to higher and higher processing centers in the brain, they continue to be compared with larger and larger swaths of the surrounding scene. This “opponent process,” as scientists call it, means that color and brightness are always relative.
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