Above: Image © wundervisuals, iStockPhoto.com

This article was reviewed and updated by the CurioCity team in September 2017.

What colour are your eyes? Brown, blue, green, hazel? What about your best friend’s eyes? And your science teacher’s?

Maybe you like your eye colour, maybe you don’t. But why are your eyes coloured in the first place? For that matter, why aren't everyone's eyes coloured the same?

Let's start by looking at what makes your eyes coloured.

Eye Colour

The coloured part of the eye is the iris, the region that surrounds the pupil (the dark center of the eye). The iris contracts and expands, making the pupil smaller or bigger, to control the amount of light entering the eye.

Did you know? The plural of iris (irises) can also be spelled irides.

The iris consists of several parts. The two important ones for eye colour are the anterior (or surface) layer called the stroma, and the posterior (or underlying) layer called the iris pigment epithelium (IPE) layer. The pigment is actually melanin, a brown pigment. It’s the same stuff that gives some people’s skin that nice tan in the summer.

It turns out that everyone produces the same amount of melanin in the IPE, but not in the stroma. In fact, it's the amount of melanin in the stromal layer that helps to determine your eye colour.

Now it may seem odd that a brown pigment like melanin can give you blue (or green) eyes. But it's true. It has to do with the fact that melanin absorbs light.

Did you know? People once thought that different eye colours were caused by temperature variations in the brain and eyes.

When white light enters the stroma, it's scattered by collagen (protein fibres) in the iris. If there is very little melanin, the shorter blue wavelength light is reflected back and gives the appearance of blue eyes. It's the same thing that happens when sunlight is scattered by particles in the atmosphere — you see a "blue" sky.

Did you know? Rayleigh scattering is the scattering of light by particles which are much smaller than the wavelength of light.

Brown eyes have a lot of melanin. Therefore, most of the light is absorbed and very little is scattered back. When the melanin content is very high, the iris appears to be black.

Aside from melanin, scientists have also discovered a brownish-yellow pigment called lipochrome (or lipofuscin). This pigment plus melanin and variations in stromal cell density will give amber, green or violet eyes.

Why Eyes have Melanin

Having melanin in the skin is a good thing. When you are exposed to sunlight, the production of this pigment helps to protect you from the harmful effects of UV radiation. But what the heck is melanin doing in the eyes?

Well, for one thing, melanin provides photoprotection of the eye just as it does for the skin. In other words, by absorbing solar radiation, it prevents harmful high-frequency light rays from damaging the retina at the back of the eye.  

Did you know? Melanin is created from the amino acid tyrosine.

Aside from acting as an ultraviolet filter, melanin is also an antioxidant. That means it’s a chemical that combines with molecules containing an odd number of electrons. (Usually, electrons are paired, just like dance partners). These molecules are called free radicals. They can be formed by exposure to sunlight, are very reactive and can damage skin cells. Melanin works by combining with the free radicals and neutralizing them.

In closing...

If you're someone who wishes they had a different eye colour, you could blame your parents. This is because eye colour is inherited.

Did you know? Some people have a condition called ocular albinism. Their eyes completely lack melanin - and, therefore, colour.

Learn More!

Ocular Anatomy (2017)
American Association for Pediatric Ophthalmology and Strabismus

Eye colour more complex than we thought (2007)
Cosmos Magazine

Rayleigh scattering (2017)

Ocular Albinism (2017)
U.S. National Library of Medicine

Stan Megraw

Stan is a writer/researcher, a PhD graduate of McGill University and was a member of the CurioCity team for several years. As a kid he dreamed of playing hockey in the NHL then becoming an astronaut with NASA. Instead, he ended up as an environmental research scientist. In his spare time Stan enjoys working on DIY projects, cooking and exploring his Irish roots.

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