The photoreceptors translate light into electrical signals. The light absorbing molecule called retinal combines with proteins called opsin to form four types of visual pigments. Depending on the form of opsin bound, retinal absorbs different wavelengths of visible light. Retinal is related to vitamin A (all trans retinol) and is synthesised from it.
Retinal can form different isomers. When bound to opsin retinal has a bent shape called the 11-cis isomer, however when the pigment is struck by light, retinal straightens to the all-trans isomer form causing it to detach from opsin. This step is the only light dependant stage. The photochemical step initiates a series of events in rods and cones causing electrical impulses to be transmitted along the optic nerve.
Isomerisation of 11-cis retinal to all- trans retinal. Image taken from http://www.life.uiuc.edu/crofts/bioph354 /bergman/kanal/transp/rhod/e2brdstext.htm
Below are the 2D and 3D structures of all-trans retinol used to synthesis retinal.
Image taken from www.chm.bris.ac.uk/webprojects2002/schnepp/retinol3dstructure.html without permission