Rhodopsin is the name of the complex between the protein opsin and 11-cis retinal in the visual process (Figure 19.27, see also here). Absorption of light by the retinal portion of the complex isomerizes the cis-bond in 11-cis retinal to a trans-bond, forming an all-trans compound called bathorhodopsin.
Bathorhodopsin (activated form of rhodopsin) can activate transducin so that it binds GTP. The transducin-GTP complex can bind to a specific phosphodiesterase that cleaves cyclic GMP to GMP. This, in turn, stimulates a cascade of events that generates a visual signal to the brain.