The concepts of resonance important to vision

The term resonance is used in a myriad of different and important ways in vision chemistry. Many of them appear 

At the atomic level, resonance is used to describe the unique properties associated with a pair of covalent electrons exhibiting opposite spin vectors. 

At the molecular level, resonance is generally used to describe the presence of a chemical duality that contributes to a lower energy state than found in otherwise similar situations. 

There is a special case of great significance to vision. That is the duality exhibited by a chain of carbon atoms terminated by a polar atom of either oxygen or nitrogen at each end. This structure can exhibit conjugation over its entire length. 

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