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How Do Eyes Get Their Blue Color?


Image credit: Wiki edit 2 via wikimedia commons. Eye color can depend on light as much as genetics.These are the same eyes in different light.

If you like to read poems, sometimes you may find the poets prefer to describe beautiful eyes as the sea or sky, but actually they possibly do not know how accurate their description is.

In each instance, the color is regarded as the scattering of multicolored light rather than a product of pigments, therefore only the blue could be able to reach the observer. When it is clear, sunlight would scatter off molecules in the atmosphere, however it is likely to do it at short wavelengths in most cases. Compared with red, blue light could be effectively scattered almost ten times.

According to the explanation of Paul Van Slembrouck, irises of those people who have blue eyes could scatter the light back in the same way. When the light that falls on the eye is white, it means that it would be comprised of a mix of wavelengths and the most of blue would be reflected back, so other people could see it. In rooms, when light might be skewed to the red end of the spectrum, eye color could be changed, because the blue is not enough to have a dominant position any longer.

In nature, apart from humans who are able to use structural coloration, it is also widely seen in marine creatures and insects as well as birds.

As for brown eyes, physics involved would be quite simple. The front layer of the eye, also known as stroma in technical term, of brown-eyed people does have enough melanin, which is the same pigment to go for darker skin color. As most light falling on the eye is absorbed, the little left to be reflected is just brown.

In case of hazel eyes with less melanin in comparison of brown, most light is absorbed; however enough could be reflected to have some scattering with a moderate amount, thus producing a color mixed with brown and blue. Different pockets of the eye might get different concentrations so as to generate dappling effects. Green eyes stand further along this continuum and only little melanin could change the color unobvious from blue. But dark specks representing in either case would generally the rear layer, or epithelium pocking through.

As far as grey eyes are concerned, they would show a different case. Although they are short of melanin, they do get deposits of collagen in the iris. Such particles, as larger as they are, could scatter wavelengths in a much even way to release a mix of colors. It is similar to the water droplets in clouds. Instead of being reflected, enough light is absorbed to make irises look like rain-bearing stratus clouds rather than white cirrus streaks.

Many babies are short of melanin at the beginning, even if they would have brown eyes when they grow up. Their blues are much deeper in color than adults, because among the most blue-eyed grown-ups, there are some larger particles in their irises, which could scatter a range of colors, so that the blue effect would be diluted.

It is known that it is the genetics that create these colors. According to Mendelian genetics, the gene for brown eyes is dominant and blue is recessive. Other colors are typically abandoned at all. However as a matter of fact, it is the multiple genes that decide eye color, although it is thought that blue eyes could be traceable to a single ancestor. The neighboring OCA2 and HERC2 genes are mainly responsible for determining melanin levels, however others are thought to be involved in factors such as collagen presence.