When light falls on the retina at the back of the eye it activates receptors that convert the light energy into nerve signals, which are transmitted along the optic nerve to the brain. The details are astonishing. There are over a hundred million of these receptors – in each of our eyes! They come in two different forms, rod-shaped and cone-shaped. Both forms contain photosensitive organic molecules (see below) which play subtly different roles. Rods are more numerous (120 million) and more than 1000 times more light-sensitive; but not to color. They are responsible for how well we can see – and why we cannot distinguish colors – at night and in low light. In bright light they are “bleached,” i.e. deactivated.
Cones, by contrast, come in three varieties, distinguished by their different “response curves” to bright light of different wavelengths (Figure), each sending different, “color-coded” nerve signals to the brain. The colors we perceive are a dynamic summation of these signals, which themselves depend on the intensity and wavelength of the incoming light, from a large number (of the 6 million cones available) of these three receptors. (The absence or deficiency of one of them is a primary cause of color-blindness.)
which show maxima at different wavelengths in the visible spectrum.
Though the color-sensitivities of the rods and the three sorts of cone differ, they all depend on the same photo-reaction of the same molecule. 11-cis-retinal is covalently bound (as a Schiff base) in each case to a different (opsin) protein, which provides a specific environment that defines the specific color response of the chromophore.
A quantum (h) of light energy is enough to convert 11-cis-retinal (derived from vitamin A) to the photo-insensitive trans form. This drastic change of molecular shape triggers a controlled series of changes that include sending the color-coding signal to the brain, and the release of the (now ill-fitting) trans-retinal molecule from the protein. The trans-retinal is economically recycled by enzymes to regenerate the 11-cis-isomer.
The recycling process is rapid for cones, which adapt quickly to changes in the incident light; but much slower for rods, which can take over half an hour to adapt after exposure to bright light; which is why it can take so long to acquire optimal night vision in this situation.
References. Color is an ideal topic for the web, and you will find many excellent treatments.
e.g. at http://www.webexhibits.org/causesofcolor/. An excellent book, which all good libraries should have, is "Bright Earth. The Invention of Colour" by Philip Ball. ISBN 978-0-099-50713-0
