Why Blue Eyes Aren’t Really Blue – 8/30/2023 – Equilibrium

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

There is nothing of that color in space where clouds float at midday in summer, nor in the irises of about 8% of the world’s population – the portion known to have “blue eyes”.

What exists, in fact, is an absence of color — and from this absence, the illusion of blue arises. This lack of pigment, combined with a physical phenomenon, makes it appear blue.

To understand better, you need to delve into the colored part of the eye.

The iris is composed of two layers: the epithelium at the back and the stroma at the front.

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

Those dark spots that some people have in their eyes are the apparent epithelium.

Against this brown background is the stroma, a delicate interweaving of colorless collagen fibers.

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

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

But not the only one.

A ‘disrupted gene’

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

But, generally speaking, there is undoubtedly a color that dominates the others.

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

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

But, generally speaking, there is undoubtedly a color that dominates the others.

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

The stroma of your 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.

Darker ones are most often seen in Southeast Asia, East Asia and Africa, and lighter ones in Western Asia, Europe and the Americas.

The curious thing is that, according to experts, for millions of years all humans had brown eyes.

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

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

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

This means that the top layer of the iris is translucent, but we are still left with the question that there is no other pigment to color it.

So where does the 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 Tyndall effect is the scattering of light by small particles floating in a liquid solution.

The fiber 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 is scattered more than other colors because it travels in shorter waves. This is why the same pair of blue eyes can appear more strongly colored at certain times than at others; their color depends on the quality of light where they are located.

The blue iris is an example of what scientists call structural coloration, as opposed to pigmentary coloration.

It is an amazing phenomenon where material appears to be a certain color without any pigmentation of that color.

In fact, a lot of times when you see blue in the natural world, what’s happening is just a structural coloration.

This is the case, for example, of the blue macaw (Anodorhynchus hyacinthinus), whose feathers are not blue, although they look that way because the nanochannels present in the feather structure alter their light-reflecting properties.

So, while brown eyes are an evolutionary marvel, as melanin protects us from the sun, blue eyes are the result of an ancient genetic mutation and a surprising phenomenon.

And the green eyes?

Well, he has a bit of both phenomena.

The stroma contains a small amount of melanin which gives it a light brown hue, but as there are translucent parts, the eye scatters incoming light and mixing colors creates the green hue.

Green eyes, despite being very common in places like Ireland and Scotland, are the rarest: it is estimated that only 2% of the world’s population has them.

A little more, 3%, have gray eyes, and they are also a curious case.

It is known that they are similar to blue ones because they do not contain melanin in the stroma.

They are thought to be gray rather than blue because they have excessive deposits of collagen in the stroma that block Tyndall’s tendency toward blue light.

This causes all wavelengths of light entering the iris to be scattered and reflected equally, creating a uniform gray hue. So if blue eyes aren’t really blue, gray eyes aren’t gray either.

Seeing is believing? Not always.