Wednesday, May 16, 2012

Blue Eyes Not Really Blue: The Tyndall Effect

I always wondered why the sky, the ocean, and eyes, that are suppose to be pigment-less, are blue. Also, blue eyes seemed to be bluer if there are any significantly blue items nearby, especially including blue apparel on or near the person. I made this diagram to help explain something called the Tyndall Effect, named after 19th century physicist, John Tyndall.

"A blue iris in an eye is due to Tyndall scattering in a turbid layer in the iris. Brown and black irises have the same layer except with more melanin in it. The melanin absorbs light. In the absence of melanin, the layer is translucent (i.e., the light passing through is randomly and diffusely scattered) and a noticeable portion of the light that enters this turbid layer re-emerges via a scattered path. That is, there is backscatter, the redirection of the lightwaves back out to the open air. Scattering takes place to a greater extent at the shorter wavelengths. The longer wavelengths tend to pass straight through the turbid layer with unaltered paths, and then encounter the next layer further back in the iris, which is a light absorber. Thus, the longer wavelengths are not reflected (by scattering) back to the open air as much as the shorter wavelengths are. Since the shorter wavelengths are the blue wavelengths, this gives rise to a blue hue in the light that comes out of the eye.[2] The blue iris is an example of a structural color, in contradistinction to a pigment color." Wikipedia

Lastly, if there is anything blue in color near blue eyes, like a shirt, or the sky, or painted wall, the amount of visible blue light waves will be increased, and those blue waves will enter and 'scatter' out of blue eyes, making them appear even more blue to any watchful eye or camera lens.

James C. Collier


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