BLOG: The surprising story of how blue eyes are like motorcycle smoke
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The color of a blue iris is not produced by so-called pigment coloring — the simple form of selective color absorption that produces the hue of everyday objects like blueberries or blue jeans.
Pigment coloring depends on pigment molecules, but blue eyes contain no blue pigment. Instead, blue irides get their color because of how their microscopic structure interacts with visible light, a process called structural color.
Irides have four layers: the anterior border layer, stroma, anterior epithelium and posterior (or pigmented) epithelium. The color of our eyes is primarily created by the interaction between the stroma and the pigmented epithelium. As light passes through an iris that appears blue, some of the wavelengths are scattered by the collagen fibers of the stroma.
Shorter, higher-energy wavelengths (blue) are scattered far more than other colors. The light that passes through the stroma, now shifted toward red, is largely absorbed by the pigmented epithelium and is not reflected back. Therefore, the wavelengths visible to an observer are overwhelmingly blue (Mason CW). This phenomenon is called the Tyndall effect.
The Tyndall effect was discovered and named by the 19th century Irish scientist and mountaineer John Tyndall. The effect is seen when light is scattered by very fine particles, such as in a suspension. The particle diameter must be between 40 nm and 900 nm, smaller than or similar to the wavelength of visible light itself.
The iris stroma is dominated by extremely delicate collagen fibers, which in turn are composed of bundles of fibrils less than 100 nm in diameter. These collagen fibers are so loosely arranged that their composition resembles a colloidal suspension, the classic situation producing the Tyndall effect. The Tyndall effect always scatters blue light much more strongly than other wavelengths and explains the blue color of smoke emitted by motorcycles and cigarettes.
Elizabeth Taylor was famous for her violet eyes, though in actuality they were more of a deep blue that could be subtly altered by framing them with her black hair and her favorite purple eye shadow.
In contrast to blue irides, the stroma of an iris that appears brown is heavily carpeted in melanocytes; the melanin absorbs light by normal pigment coloring and reflects back brown. Green eyes are otherwise blue irides but with a small amount of melanin, which shifts the blue to a darker green color. A hazel eye is a mixture of brown and green or brown and blue; the brown pigment always advances centrifugally from the pupil, and no human has ever been observed with a central green ring and a peripheral brown area (Mackey DA, et al).
Geneticists believe that blue eyes are a recent phenomenon and that all human eyes were brown as recently as 7,000 years ago. A single base mutation of thymine to cytosine within the HERC2 locus is almost perfectly associated with blue eyes. The greatest incidence of blue eyes is in the countries surrounding the Baltic Sea; evidence suggests that all blue eyes in the world originated from one single ancestor who lived in that area (Eiberg H, et al).
The future of blue eyes is unclear. Since blue eyes are a recessive trait, requiring inheritance of the gene from both parents, time and genetic mixing inexorably cause the frequency of brown eyes to increase throughout the population. In 1900, 57% of white individuals in the U.S. had blue eyes; today this percentage has dropped to 17% (Grant MD, et al).
It appears inevitable that the incidence of blue eyes will dwindle with each generation, perhaps ultimately one day to zero. There may come a time when the era of blue eyes will be remembered as a brief but beautiful time in our history.
References:
- Eiberg H, et al. Hum Genet. 2008;doi:10.1007/s00439-007-0460-x.
- Grant MD, et al. Ann Hum Biol. 2002;doi:10.1080/03014460210157394.
- Mackey DA, et al. Clin Exp Ophthalmol. 2011;doi:10.1111/j.1442-9071.2010.02487.x.
- Mason CW. J Phys Chem. 1924;doi:10.1021/j150239a007.
For more information:
Oliver Kuhn-Wilken, OD, practices at Pacific Cataract and Laser Institute’s Tualatin Clinic in Oregon.
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