The fascinating facts behind the phenomena of eye colours

Werner Goldberg, a half-Jew, was the blue-eyed poster boy of the Nazis. Well, quite literally. He was portrayed as an icon of the ‘ideal German solider’, on the Nazi propaganda posters.

The coloured part of the eye is the iris. For far too long, its colour has commanded a lot of human attention. From being a basis to Nazi eugenics (and determining the fate of countless lives) to enrapturing the whole world on the cover page of the National Geographic magazine, eye colour has always fascinated mankind. Some people find it to be a horoscopic trait, others a personality-definer, while yet others believe it to be imbued with mystical qualities and magical powers.

Nature abounds with biological pigments, the natural colouring materials of tissues. Green leaves owe their colour to a complex compound called chlorophyll, that enables them to absorb energy from sunlight. Ageing leaves take on an orange hue due to decreasing chlorophyll and increasing xanthophyll (yellow) and carotenes (red-orange) pigments. Anthocyanin makes pansies purple. The pigment luciferin underlies the characteristic glow of a firefly. Consumption of carotenoid-rich shellfish imbues flamingos with their salient pinkish tinge.

In human beings, complexion is determined largely by melanin, a protective pigment that shields the body from solar ultraviolet radiation. The pigment is also responsible for determining hair colour, and its agglomeration results in moles. It makes up squid and octopus ink. There are mainly two variants of melanin (besides the rather obscure neuromelanin): eumelanin and pheomelanin, which are brown-black and pink-red respectively. Eumelanin is of two subtypes: black eumelanin and brown eumelanin.

The colour of a pigment is determined by the wavelengths it reflect back. For instance, chlorophyll absorbs the red and blue parts of the spectrum, while reflecting back the green light. Similarly, black eumelanin appears so as it

absorbs all wavelengths almost equally. Because melanin is found in most external tissue as skin and hair, one might presume it gives eyes their colour.

Indeed, melanin is the pigment found in irises. Black eumelanin leads to black eyes, and varying concentrations of brown eumelanin give a range of brown eyes. So far, so good. Now what about blue and green eyes?

One might be tempted to believe that they result from the presence of coloured pigments in the iris. But let’s tinker with the presence of such a pigment in the human body. Pause and ponder. Do we recall any other instance of a green or blue colouration in the human body? Bile and gore appear greenish due to biliverdin, a green pigment produced by the breakdown of blood.

Such a transient, destructive and gruesome process is unlikely to underlie eye colouration. Now as for blue, it is a colour difficult to find among birds and beasts, as well. There are hardly any plants that have a true blue colouration.

That brings us to our second determinant of (apparent) colouration: structure. Besides pigmentation, structure and dispersion of any membrane, matrix or fibre contribute to its colour. The elegant blue morpho and the stark blue jay owe their distinctive colouration to the optical phenomenon of scattering. Budgerigar parrots occur in a host of colours: yellow, cyan, green. It gets its yellow colour from psittacofulvin pigment, while the azure ones lack any pigment altogether. How do the azure and green ones get their colouration? We’ll get back to structural colouration shortly.

An iris is composed of two parts: the epithelium at the rear, and the stroma at the front. If you’ve closely observed an eye, you might have noticed little flecks, specks and strands in the iris – that is the epithelium, a two-cell thick layer. The stroma layer is composed of colourless collagen fibres. This mesh appears as either annular (appearing as concentric circles) or radial. The epithelium contains melanin while the stroma may or may not contain melanin.

Black or brown eyes simply have a high amount of melanin in the stroma that absorbs all colours incident upon it. Blue eyes are actually pigment-free. They have little or no melanin. So why don’t they appear colourless? The answer lies in the same phenomenon that gives the sky its colour. Blue light travels as

shorter waves, and hence is scattered more than other colours. When the sun is overhead, the blue colour pervades the ambience owing to scattering and thus dominates other colours. At dawn and dusk, red prevails, as blue is scattered away in other areas (beyond the sight) due to the light travelling a greater distance. Because the low sun is at a small angle (slant) to the horizon, blue gets dissipated early on, before it can reach the observer, lending the sky a crimson hue. This optical phenomenon is called ‘Rayleigh Scattering’.

Put simply, it entails very small particles scattering colours of the shorter (blue) end of the spectrum much more. A related phenomenon is the ‘Tyndall Effect’, in which particles that are finely dispersed in a liquid but not dissolved (called a colloid), scatter shorter wavelength colours more strongly than others. An everyday example is the bluish tinted smoke of dilapidated or inefficient motor engines (in which incompletely burnt fuel soot serves as particulate matter).

Melanin provides opacity. In its absence, the stroma is translucent, meaning it transmits some light and scatters the other portion diffusely. When there is lack of sizeable absorption, ‘backscatter’ occurs, that is, a substantial portion of the light entering the eye is redirected and scattered out of it. Because longer wavelengths such as red and yellow are scattered insignificantly, they pass undeviated and get absorbed in the melanin-rich black epithelium. On the other hand, the scattered blue makes it out of the eye and reaches out. The eyes perceive the eye colour of the other person as blue. So much so for a fairly recent evolutionary trait – blue eyes evolved in humans a few ten thousand years ago, likely from a single individual (all blue-eyed people probably share a single common ancestor)! So much so for a trait that used to earmark survivors and victims in one of humanity’s worst genocides. So much so for a colour that is seldom encountered in nature. Until recently, blue was incredibly difficult to procure and was obtained solely by grinding specific minerals.

Returning to the question of blue in nature, butterfly wings are essentially colloids, a dispersion of solids in solids. The famed blue morpho is completely colourless. It essentially utilises scattering to appear blue, through the translucent-coated scales comprising its wings. Butterfly colours are predominantly structural colours. To tell a structural colour from a pigment, one needs to simply view it from various angles and observe if it varies. Together with bird feathers, butterfly scales are instances of ‘iridescence’. The structure of their layers is inconsistent, making them appear to be different

when viewed from different angles. This makes butterflies appear as ephemeral, flashes as if they were teleporting or morphing, to evade predators. This is similar to what happens in lenticular printing — for prints where two or more different images can be seen when viewed from different angles.

The blue-coloured budgerigar, similarly, lacks the yellow psittacofulvin pigment and merely scatters the light. The green budgerigar possesses moderate amounts of the yellow pigment, whose reflected yellow light combines with the backscattered (as its feathers are not rendered opaque by full pigmentation) blue to yield an effective green.

Grey eyes, as in the case of blue ones, have a dark epithelium and a clear stroma. The difference is in the amount of collagen in the stroma. Grey eyes are loaded with extra collagen. The copious structural protein obscures scattered light. Its fibrous structure obstructs the usual scattering in the same way raindrops (which are larger than fine dust and particulate matter) turn the sky grey. Rayleigh scattering needs very small particles. Rain droplets do not meet this criterion, and when dispersed in the atmosphere they interfere with the blue scattering. Similarly, the collagen fibres render the eye turbid. Grey eyes have vagrant shades – they appear to change with lighting, ambience, contrast (even eyeliners or other makeup in their vicinity might make them appear different), and mood – exerting different pressure on the iris and leading to different colouration. Green eyes result from the presence of a small to moderate amount of melanin combined with Rayleigh Scattering. Green eyes are varied because the melanin and the scattering colours combine differently and are themselves individually quite varied. Hazel eyes are similar to green ones in most respects, except in possessing a moderately high amount of melanin in the border layer of the iris. The inconsistencies in the distribution in stroma give sporadic zones of green and yellow. Hazel eyes appear to shift colour from brown to green depending on the lighting.

Amber eyes markedly differ from hazel ones in possessing lipochrome, which is a brownish-yellow pigment, rather than having a yellowish-brown structural colour. This can be ascertained by varying the lighting or the semblance —

amber eyes stay the same under a lamp or an LED bulb-lit room, as they do under the sun.

Meanwhile, because hazel eyes partly depend upon scattering from their translucent layer, different lights scatter differently and yield varying final colours. Structural colours simply deflect a portion of the incident light and hence depend on it to a large extent. Pigments, however, reflect only a fixed colour, irrespective of the ambient lighting. Eyes with moderate lipochrome appear green as the yellow reflected light combines with the scattered blue light. Finally, albinos lack melanin altogether. This also means that besides their stroma, their iris epithelia lack melanin too. This epithelium is responsible for absorbing longer wavelengths in colourless eyes. When melanin is absent, the fine blood tubules behind the epithelium become apparent as the retina is visible. These superficial capillaries, which supply the eyes with blood, make albino eyes appear pinkish-violet when the reddish colour combines with scattered blue. Under certain lighting conditions, deep blue eyes can appear violet, as with those of Elizabeth Taylor.

Eye colour might seem a superficiality, but it’s more than skin-deep. Each eye colour seems to have a personality of its own – blue being the most deceptive. With a qualitatively different basis to each hue, it seems naïve for humans to base anything upon them as a criterion. Yet, from ancient to modern times, eye colour has carried myriad connotations — mystical and supernatural ones in early times to personality analytical in more modern days.

Besides other criteria, eye colour was the key to obtaining a precious and prestigious “Aryan certificate” in Nazi Germany. It determined whether forced abortions would be imposed. It determined one’s status of ‘German descent’. It granted for pregnant women admission into Lebensborn, a secret Nazi pure-breed perpetration programme majorly mostly availed by single women. Blonde “Nordic-looking” women were encouraged to bear babies with SS officers as part of an ambitious expansion plan of the “master race”.

Nordic features such as blond hair and blue eyes were idolised, and eugenics was practised to preserve the traits. If you were a Polish or Russian child in the late 1930s, this ambience-dependent trait could determine whether you would be kidnapped and raised in aristocratic foster families, or dispatched to labour and/or extermination camps. Later, blue-eyed children in Nazi-occupied

nations as well as from neighbouring territories were abducted and raised in pure-blood families. Nazis ‘researched’ and wrote volumes on racial superiority, eugenics and the ‘scientific’ study of race. Their historical and biological theorisations seldom had any scientific basis. Speculations and hypotheses were propagated as sacrosanct facts.

In 2016, research put the earliest known blue-eyed specimen at 7,700 years ago. Had the Nazis actually conducted some scientific research, they would have realised their “superior traits” were but mutations in a singular individual, and that humans all had African origins, with dark skin and eyes. However, eye colour is determined by variation at several different genes and their mutual interactions, which make it possible for two blue-eyed parents to have brown-eyed children. Alas, genetics was still ill-developed and superficially interpreted, and the intricate bases that underlay eye-colour determination were well beyond reach. It is a testimony to how fervour can couple with half-knowledge, to destructive consequences. The Nazis obsessively and arbitrarily assumed a detailed basis to genetics, with little causative understanding.

However, had an entire civilisation not had their eyes clouded by bigotry, not melanin, they would have at least observed that blue eyes were not a trait simply inherited as such; that they did not characterise a race; that neither were two blue-eyed parents a guarantee for a blue-eyed offspring, nor were two dark-eyed parents a preclusion to the birth of a blue-eyed child.

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