The light-sensitive pigment found in the rods of the vertebrate retina. It consists of a protein component, opsin, linked to a nonprotein chromophore, retinal (or retinene), a derivative of vitamin A. Light falling on the rod is absorbed by the retinal, which changes its form and separates from the opsin component; this initiates the transmission of a nerve impulse to the brain via an intracellular signalling cascade involving the G protein transducin. The great sensitivity of rhodopsin allows vision in dim light (night vision). See also dark adaptation.