How Do Blue Eyes Get Their Color?
Geege Schuman stashed this in Science Too
Brown eyes have simpler physics than blue eyes.
When poets compare beautiful eyes to the sea orsky they're probably not aware how accurate they are being.
In each case the color is a product not of pigments but the scattering of multicolored light so that only the blue reaches the observer. On a clear day sunlight scatters off molecules in the atmosphere, but does so more at short wavelengths. Blue light is scattered almost ten times as effectively as red.
As Paul Van Slembrouck explains in this Mediumpiece (with drawings) irises of people with blue eyes scatter the light back in the same way. If the light falling on the eye is white, that is contains a mix of wavelengths, it is mostly blue that will be reflected back for others to see. Indoors, with light that may be skewed to the red end of the spectrum, eye color may seem to change, since there is no longer enough blue to dominate.
Brown eyes have much simpler physics. The stroma, or front layer of the eye, of brown-eyed people has plenty of melanin, the same pigment that makes for darker skin color. Most light falling on the eye is absorbed, and we see the little that is reflected as brown.