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Why do humans develop convex lenses in their eyes and not concave lenses?
In order for an eye to work well to give a brain information about the
world, there needs to be some simple way for the brain to make sense of
the nerve impulses coming from the eye. It's hard to see how that could
work unless the signal from some object triggered a specific set of
nerve endings. The easiest way to do that is to focus the incoming
light. That way, light coming from one direction (say the direction of
an approaching lion) gets focussed onto one part of the retina- the
back of the eye where the nerves come in. Each part of the lion sends
some light to the observing eye, and these get focused down to
different, close-by points on the retina, forming a clear image of the
Concave lenses spread the rays out, so they would land all over
the retina, leaving the retina only sensitive to the average light
level and average color of everything in the field of view. The brain
cannot decode more information from this, and cannot identify the
images of food or predators. So if there were any creatures whose
lenses happened to be concave, they left no descendants. Of course
these same design issues came up long before there were people. So far
as I know, all animals with lenses have convex lenses.
Mike W. (and Tom J.)
(republished on 07/29/06)
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