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Q & A: eye lenses

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Most recent answer: 10/22/2007
Q:
Why do humans develop convex lenses in their eyes and not concave lenses?
- Emily
New York
A:
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 approaching lion.

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.)

(published on 10/22/2007)

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