It's amazing what happens in an eye: retinal (C₂₀H₂₈O), an aldehyde of Vitamin A, exists in two forms: all-trans-retinal gets hit by photos, flips and becomes 1

It's amazing what happens in an eye: retinal (C₂₀H₂₈O), an aldehyde of Vitamin A, exists in two forms: all-trans-retinal gets hit by photos, flips and becomes 13-cis retinal (simplifying slightly). If enough photons are involved, the info gets transmitted to the train. If the photons were in a particular constellation, we see a "rectangle" or "circle"; quite a miracle if you think about it.