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Why Blue Eyes Aren’t Really Blue |  Sciences

Why Blue Eyes Aren’t Really Blue | Sciences

Why blue eyes aren’t really blue – Image: Getty Images via BBC

When it is said that someone’s eyes are blue as the sky, it is true: the sky and the eyes are not blue at all.

There is none of this color in the space where clouds float at midday in summer, nor in the irises of about 8% of the world’s population – the part known as “blue eyes”.

What is there, in fact, is the absence of color – and from this absence arises the illusion of blue. This lack of pigment, plus a physical phenomenon, makes it appear blue.

For a better understanding, you need to delve into the colored part of the eye.

The iris consists of two layers: the epithelium in the back and the stroma in the front.

The epithelium is only two cells thick, and in almost all cases, even in blue eyes, it contains dark brown pigments.

Those dark spots that appear in some people’s eyes are the epithelium.

Against this brown background is the stroma, which is a fine tangle of colorless collagen fibrils.

The stroma often contains melanin, the substance in the body that also produces pigmentation in the skin and hair.

The abundance or deficiency of this pigment is a determining factor in eye color.

Eye color is a unique part of personality characteristics. Apparently, no two people in the world have eyes of exactly the same color.

About 80% of the world’s population has some variation in brown around the pupil.

The stroma of the iris contains a high concentration of melanin, which absorbs most of the light and creates shades of brown ranging from chocolate to amber to hazel.

Dark colors are most often seen in Southeast Asia, East Asia, and Africa, while lighter colors are seen in West Asia, Europe, and the Americas.

And the strange thing is that according to experts, for millions of years all humans have had brown eyes.

But between 6,000 and 10,000 years ago, “a genetic mutation occurred that affected the OCA2 gene (involved in melanin production) in our chromosomes,” explained the director of the original study, Hans Eiberg, of the University of Copenhagen in 2008.

This mutation created a “genetic switch” that turned off that progenitor’s body’s ability to produce brown eye pigment.

Therefore, all blue eyes go back to an ancient common ancestor, from whom the property of having no melanin in the stroma was inherited.

This means that the top layer of the iris is transparent, but we still have the question that there is no other pigment to color it.

So where does blue come from?

Blue…and even green and gray

The explanation for why some eyes are blue is the same as why the sky is blue: a phenomenon known as the Tyndall effect.

The fibrous structure of the stroma scatters light in a similar way, tending to scatter short-wavelength light more than long-wavelength light.

When visible white light hits it, blue spreads out more than other colors because it travels at shorter wavelengths. This is why the same pair of blue eyes can appear a stronger color at certain times than at others; Their color depends on the quality of the light in the place where they are located.

Blue irises are an example of what scientists call structural, rather than pigmentary, coloration.

It is an amazing phenomenon where the material appears a certain color without any pigmentation for that color.

In fact, often when you see blue in the natural world, what’s going on is just a structural coloration.

This is the case, for example, in the hyacinth macaw (Anodorhynchus hyacinthinus), whose feathers are not blue, although they appear to be because nanochannels in the structure of the feathers change their reflective properties.

So, while brown eyes are an evolutionary marvel, with melanin protecting us from the sun, blue eyes are the result of an ancient genetic mutation and an amazing phenomenon.

Well, he has a little of both phenomena.

The stroma contains a small amount of melanin which gives it a light brown colour, but because it has transparent parts, the eye scatters the incoming light and mixing of the colors creates the green colour.

A little more, 3%, have gray eyes, which is also a strange condition.

They are known to resemble blue ones because they do not contain the melanin present in the stroma.

They are thought to be gray rather than blue because they have excess deposits of collagen in the stroma which inhibit Tyndall’s tendency towards blue light.

This causes all wavelengths of light entering the iris to be scattered and reflected evenly, resulting in a uniform gray colour. So, if blue eyes aren’t really blue, then gray eyes aren’t gray either.

seeing is believing? not always.