Let's now take a closer look at how light and retinene interact. The diagram at left shows a retinene molecule with it's carbon atoms numbered. Notice the alternating single and double bonds that begin at carbon #5 and continue through #15.

Referring to our description of cis and trans isomer, you may correctly recognize that there are many isomer possibilities amongst all these double bond pairs in this conjugated system. There is one pair of carbon atoms that is particular to vision This is the pair of carbons that form the double bond at 11 and 12.

Wald discovered that the cis isomer at carbons 11 and 12 of retinene fits nicely into a point of attachment of the protein, opsin. This complex of retinene with opsin forms a rhodopsin.

A photon of light is absorbed by the pi electrons in the conjugated chain of the retinene. This higher energy level flips the cis isomer to the trans position, straightening out the molecule. The straightened trans-retinene no longer fits the opsin protein and comes away. The deeper colors of the original pigment is replaced by the yellow color of free retinene. This process is sometimes referred to as a "bleaching" of the visual pigments.

It is this process or isomerization that forms the "trigger" that begins the events that lead to vision. It's the mechanism by which the energy contained in a quantum of the suns energy can be transferred to the biochemical world of our bodies.