Every time you see something, light is flipping a double bond from cis to trans at the back of your eye.
The back of your eye is lined with the retina, a sheet of light-sensing cells. Its rod cells, which let you see in dim light, are packed with a protein called rhodopsin. Tucked into a pocket inside each rhodopsin sits one small molecule: 11-cis-retinal.
Retinal is a form of vitamin A, with vitamin A's -CH₂OH group oxidised to an aldehyde group, -CHO. It has a run of alternating double bonds, and held in the protein's pocket it absorbs blue-green light, around 500 nm. Its name tells you its arrangement: the double bond between carbons 11 and 12 is cis, so the chain has a sharp bend there.
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Light flips the bond
When retinal absorbs light, the light's energy makes an electron of the second shared pair jump, and for a moment the second bond lets go. The end of the chain swings round the bond between carbons 11 and 12, and the double bond locks again in the trans form. In about 200 femtoseconds, 0.0000000000002 seconds, the bent 11-cis-retinal has become straight all-trans-retinal.
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The straight molecule fits its pocket badly, so the rhodopsin around it changes its own folded arrangement. The changed protein sets off a chain of reactions in the rod cell, and a signal travels through the retina and along the optic nerve to your brain. A rod cell can respond to a single particle of light.
The all-trans-retinal then leaves the protein. Cells at the back of the eye turn it back into 11-cis-retinal, which settles into a fresh pocket, ready for the next flash. The cone cells, which see colour, use the same 11-cis-retinal in three other proteins, each tuned to its own range of colours.