Why are my eyes blue?

By Denise Croote '16

What do Zac Efron, Matt Bomer, and Megan Fox have in common with the baby depicted to the right? They all have vividly blue eyes and can thank the wondrous properties of Rayleigh scattering for it.  

The function of the iris is to limit the amount of light that passes through the lens to the retina. In order to do this, the iris is coated with a pigment called melanin. Depending on the concentration of the melanin present, the pigmentation of the iris in humans will vary from black to light brown (1). How then do some individuals have blue or green eyes if neither blue nor green pigments are present in the iris?

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This is possible because the iris consists of three layers: a thin top layer, a back layer, and a spongy layer in-between called the stroma. Individuals with brown eyes have melanin present in the stroma, which allows for light of both short and long wavelengths to be absorbed. Individuals with blue eyes have brown pigment only on the back layer of their iris and lack pigment in the stroma. This pigment on the back layers absorbs longer wavelengths of light and reflects shorter wavelengths of light. When light enters the eyes of blue-eyed individuals, it is reflected and scattered by small particles suspended in the stroma. This is known as Rayleigh scattering and also explains why eye color for lighter-eyed individuals appears to shift when the lighting changes (2).

Babies are born with blue eyes because they initially lack melanin. Within the first year of life, the baby’s melanocytes continually produce pigment and the color of the eyes can change from blue to green or brown (1).  Green eyes are a combination of the effects of the light brown pigmentation of the stroma and the Rayleigh scattering of blue light.