Color Vision 
When light hits the human eye, it is focused by the lens and captured by the retina, the nerve tissue which lines the posterior portion of the eye. The retina contains two different types of light sensitive receptors: rods and cones. Rods depict black and white while cones detect colors. There are about 125 million rods and 6 million cones in the human eye. In addition to cones, ganglion cells line the retina and assist with color vision.
There are two competing theories as to the scientific explanation for human perception of color. The first theory is commonly referred to as the Opponent Process Theory or Hering Theory. This theory states that there are only two cones present to detect color. The first is a blue/yellow cone while the other is a red/green cone. Both cones work on an opponent process. This means that if the blue wavelength receptor is on then the yellow color receptor is off and visa versa. The same holds true for the red/green cone. This theory is advantageous because it solves the problem of seeing bluish yellow and reddish green colors which is physically impossible yet predicted by the opposing color perception model, Young-Helmholtz Theory. The problem with Hering’s theory is that it does not predict a variety of colors which is present in the real world.
The Young-Helmholtz Theory, also known as the Trichromatic Theory, proposes that there are three cones in the retina of the eye. Each cone is receptive to different wavelengths, which accounts for the different color each cone detects. The first cone detects blue colors and is most receptive at short wavelengths. The second cone detects yellows and operates best at medium wavelengths while the final cone detects red colors and is sensitive at long wavelengths. A single wavelength will stimulate all three types of cones. However, it is the degree to which each cone reacts and the strength of their signal that determines the amount of each color that is perceived. Ganglion cells, which also line the retina of the eye, help in this process by comparing signals from the different cones and determining the amount of each color. The Young-Helmholtz Theory predicts the variety of colors we see and attributes the ability to view such colors to the three cones: yellow, blue and red.
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