a chromoprotein (protein linked to a pigment-carrying substance) that is contained in the light-sensitive cells of the rod type in the retina of the eye; it functions in the eye's adaptation to dim light. When the eye is exposed to bright light, the rhodopsin bleaches; after an interval of darkness, it returns to its former purple-red colour.

The pigment-bearing portion of rhodopsin is retinal, a substance formed by oxidation of vitamin A. The protein portion is opsin. In a bright light rhodopsin breaks down into retinal and opsin; in the dark the process is reversed. See also rod.