This posting clearly describes rhodopsin and signal transduction. It also cites references.
There are a lot of great sources online that give brief explanations of rhodopsin and signal transduction.
A good summary from Wikipedia regarding what rhodopsin is:
"Rhodopsin, also known as visual purple, is a biological pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light, enabling vision in low-light conditions."
Wikipedia cited the following for this:
Litmann BJ, Mitchell DC (1996). "Rhodopsin structure and function". In Lee AG. Rhodopsin and G-Protein Linked Receptors, Part A (Vol 2, 1996) (2 Vol Set). Greenwich, Conn: JAI Press. pp. 1-32. ISBN 1-55938-659-2.
A good explanation of signal transduction and rhodopsin ...
Rhodopsin and signal transduction are key concepts in this solution.