In Casablanca, Rick is called over to the table of Major Strasser, the diabolical Gestapo officer who’s been keeping a dossier on him. Rick glances at Strasser’s notebook and quips, "Are my eyes really brown?"
Had Strasser been a scientist, he would have answered with a resounding yes! As to the answer of the question I posed in the title above, the answer is no. You could have fooled me, but then, I’m easily fooled. And so, it seems, are you… As it happens, all human eyes are pigmented brown!
Eye colour is perceived in the iris, the circular layered structure that controls the size of the pupil. The epithelium—a layer that is just two cells thick—sits at the back of the iris, and the stroma is in front. Those dark flecks that you can sometimes see in your eyes are the epithelium showing through the structure of the stroma—the epithelium contains brown or black pigment.
The stroma is composed of colourless collagen fibres. It can contain melanin too, which is that brown pigment; or additional deposits of collagen. And that’s it! Brown pigment or colourless filler. No blue or green or hazel. So what’s going on with my sky blue eyes?
People like me have a mostly colourless stroma. Not much pigment or excess collagen. Light entering my eyes is scattered, which creates a Tyndall effect (where light is scattered by particles in a fine suspension). Longer wavelengths (red to yellow) are absorbed by the iris, shorter wavelengths (at the blue end of the spectrum) are reflected back, so you see blue. And that, interestingly, makes ‘blue’ eyes unique as they change their hue under different light intensities.
Green eyes have a little more melanin in the stroma, and they also have no excess collagen deposits. Some light that enters gets absorbed by the melanin but the stroma also scatters some light. Green is the combination of short wavelength scattering and brown pigmentation. Hazel is another mixture of scattering and brown pigment based on different proportions of each and some excess collagen deposits. And brown eyes contain a high concentration of melanin in their stroma, absorbing more light regardless of excess collagen.
So next time you imagine you can drown in the cerulean sea of your lover’s eyes, keep in mind, those depths are murkier than you might think.
Further Reading:
http://medicine.jrank.org/pages/2230/Eye-Color-Iris-Structure.html
