Ask A Scientist General Science Archive

name         Justin
status       educator
grade        K-3
location     RI

Question -   I understand what determines eye color, but what is the
purpose of having eye color? I mean, does the color of our eyes have any
effect on vision or color perception?
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The color in our eyes does not change our perception. However, everyone sees 
everything and colors just a little bit differently. As far as I know, eye  
color is just another distinguishing characteristic, like hair color.

Grace Fields
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I positively think iris-color of an eye has no effect on color -perception in that eye.
You can even wear tinted contact lenses, and the brain will adjust the color-balance of your perception to mostly compensate it away.


The iris must have some color because it must be quite opaque to block the light outside the pupil.
That's it's job, blocking light and changing size, all in a thin, small space.
The scattering-based white colors tissue can do are not dense enough, they are too translucent in such thin sections.
Living organisms don't usually have the ability to make metals or graphite, so they can't use that for opacity.
Adding absorbing dyes to the tissue-scattering is the only feasible way within "this technology".
The color and density of the dye apparently have room to flex, and evolution can afford to play little games with it.


An additional plausible reason is UV protection.
UV degrades tissue.  Two ways to cope are:
  - blocking the UV with a highly absorbing dye (melanin), or
  - replacing the tissue regularly.
Perhaps the muscles of the iris can be regrown regularly throughout your life, and can do without dye protection,
but we know the lens of the eye is not so lucky.
We know that a substantial percentage of our race gets clouded and stiff lenses as they get old.
We know that UV is a contributing factor to this.
Such problems might occur faster if the lens didn't have partial UV blockage by the colored iris in front of it.
(Somebody check for me whether the iris is in front of the lens, or behind?)
Especially in primitive times when more of our lives were outdoors, the times when
most of the evolution that shaped us presumably occurred.


Accidental development of a social force in the evolution is also conceivable,
but I don't think it has mattered to us enough to redirect our evolution, yet.


Jim Swenson
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