The retina is lined with many millions of photoreceptor
cells that consist of two types: 7 million cones provide color
information and sharpness of images, and 120 million rods are
extremely sensitive detectors of white light to provide night
vision. The tops of the rods and cones contain a region filled
with membrane-bound discs, which contain the molecule cis-retinal
bound to a protein called opsin. The resulting complex is called
rhodopsin or "visual purple".
The molecule cis-retinal can absorb light at a specific wavelength.
When visible light hits the cis-retinal, the cis-retinal undergoes
an isomerization, or change in molecular arrangement, to all-trans-retinal.
The new form of trans-retinal does not fit as well into the protein,
and so a series of geometry changes in the protein begins. The
resulting complex is referred to a bathrhodopsin (there are other
intermediates in this process, but we'll ignore them for now).
As the protein changes its geometry, it initiates a cascade
of biochemical reactions that results in changes in charge so
that a large potential difference builds up across the plasma
membrane. This potential difference is passed along to an adjoining
nerve cell as an electrical impulse. The nerve cell carries this
impulse to the brain, where the visual information is interpreted.