Rhodopsin

Rhodopsin is composed of retinal and an opsin molecule. The opsin sets the color sensitivity of the molecule while the retinal is the molecule that is sensitive to light. In the presence of light the retinal changes from a cis form to a trans form (also referred to as "bleached"). This happens very quickly as the rhodopsin dissociates into it's components in the presence of light. The reassembly of the opsin and retinal is relatively slow