Scientists have a good overall understanding of human vision: when light enters our eyes, it is focused by the lens and strikes the retina in the back of the eye. The light causes some of the millions of photoreceptor cells that line the retina to undergo a chemical change, which send a message through the optic nerve fiber to the brain, which creates a picture. However, there are still a few unresolved questions in the details of the vision process, one of which is why the eye evolved to use a certain light-absorbing chromophore called 11-cis-retinal instead of one of its isomers (i.e., molecules with the same atoms but in different arrangements), such as 7-cis, 9-cis, or 13-cis.
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