The mechanism of seeing in the dark involves two types of cells – rods and cones, in the eye. These cells are present in the light – sensitive innermost layer of the eye called the retina. They lie in front of a pigmented tissue layer. Cones are present in the area of greatest visual activity – fovea contrails, which lies at the centre of small yellow pigment spot behind the pupil. Rods and cones are present around the fovea.
Cones are active under intense illumination, whereas rods are active in dim light. In the dark rods are sensitized by a pigment called Rhodospin or the visual purple that is formed within the rods. Rhodospin is bleaches by light and is reformed by the rods in darkness. Hence a person who steps from sunlight into a dark room experiences a blinding feeling till the pigments begin to form. This process takes around 30 minutes to reach maximum sensitivity.
On completion the eyes become sensitive to low levels of illumination and are said to be dark-adapted. Meanwhile the cones adapt themselves to fainter light in the ambience of low intensity illumination, which may take around five minutes.
The best example is finding our way to our seats in a movie theatre after the show begins. Initially there is a blinding feeling when we do not see anything. But later the cones in the retina get adapted to the light from the film screen, when we are able to find the seats; this is followed by adaption of the rods which enables us to see everyone around us.