Tuesday May 29, 2012 Q&A

Why are lighter-colored eyes more sensitive to light?


1 Lighter-colored irises let more light pass into the retina

2 The blue pigment attracts harmful UV rays

3 Lighter-colored eyes have more higher density of nerve cells

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Answer: lighter irises let more light pass into the retina. You think people wearing sunglasses even in cloudy weather are doing it only to look cool? Well, it depends. People with lighter colored eyes need more protection from direct sunlight but also diffused light and bright, florescent interior lighting. They are more sensitive than their dark-eyed counterparts because they lack a protective pigment in the iris. Lighter eye color, such as blue or green, is due to the absence of pigment, called melanin, and not due to a different color of pigment, as could be expected. Instead, these lighter colors are due to a scattering of light known as Rayleigh scattering—the same physical process that explains why the sky is blue; when white light is diffused through small particles and refracts into different spectrums. In light colored eyes, light passes more easily through the iris' stroma—a delicate bundle of fibers and cells—and is reflected off the layer of darker cells directly behind the stroma called the epithelium. Longer wavelengths of light are absorbed by epithelium while the shorter wavelengths such as blue and green undergo the scattering phenomenon as the pass back through the stroma. This almost complete lack of pigment in people with light-colored eyes makes them more sensitive to light. For when melanin develops, it creates a protective filter that reflects the light back out of the eye. Consequently, in light eyes more light passes through the iris and can over stimulate the bundle of nerve cells at the back of the eye known as the retina. It is believed that people with blue eyes in adulthood have a single common ancestor whose genes underwent a mutation between 6,000 and 10,000 years ago in what is today Romania. Genetically, eye color is considered a “complex trait” because it is caused by a combination of multiple genes whose interaction is not completely understood. For the moment, it is impossible to determine what color eyes someone has only from their DNA with 100% accuracy. With the collaboration of Hopital ophtalmique Jules-Gonin