Tuesday, January 02, 2018

What makes blue eyes blue?

I came across a fascinating article on what makes blue eyes blue.
Like probably most people, I have always assumed that eye colour is purely the result of pigmented cells, but that's apparently not the case. It turns out that the appearance and the various different colours of eyes are the result of the two microscopically thin layers of cells that cover the iris: the epithelium at the back (which is mainly seen as the dark specks and flecks in people's irises), and the stroma at the front. The stroma is made of colourless collagen fibres, with varying amounts of dark brown-pigmented melanin, as well as some excess collagen deposits, which between them structurally create the appearance of a colour.
Brown eyes, for example, have a high concentration of melanin in their stroma, which absorbs most of the light entering the eye regardless of any collagen deposits (i.e. very little light is reflected, giving the eye a dark appearance from the outside).
Green eyes don't have much melanin, but neither do they have any collagen deposits (the melanin pigment therefore absorbs some but not all of the light entering the eyes, and scatters the rest creating a blue colour due to the Tyndall effect - essentially the same as the Rayleigh scattering that causes the sky to appear blue - the resulting combination appearing as different shades of green or hazel).
Pure blue eyes have no melanin pigment colouring at all, and no excess collagen deposits either, so their colouring is entirely due to structural colouration (all the light entering is scattered back due to the Tyndall effect). Thus, staring into someone's blue eyes is a bit like staring at the sky, which is a happy thought. Also, for this reason, the appearance of blue eyes (and, to a lesser extent, green eyes) is much more dependant on the quantity and quality of the ambient light than other eye colours, and so tend to appear much more variable in their colouration.
Even rarer are grey eyes (like mine), in which the stroma has no melanin pigmentation, but does have a small amount of collagen deposits which interfere with the light scattering (in fact, all light wavelengths are scattered evenly, yielding the grey colour).
And violet eyes? Well, in theory, violet eyes (as Elizabeth Taylor, for example, was purported to have) do not exist, and Elizabeth Taylor probably had grey-blue eyes that could appear to have a violet tint under certain lighting conditions. Damn!

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