Why is it that eyes take so long to adjust to a dark room?
When we go from a bright room into a dark one, there is an enormous difference in the amount of light that reaches our eyes - a difference of up to 10,000-fold, according to Dr. John Loewenstein, associate chief of ophthalmology at the Massachusetts Eye and Ear Infirmary."To go from a bright light to your most sensitive state in darkness takes about one hour," he says, "but until it gets really dark, we don't notice the difference."
Our eyes respond to darkness in two ways. First, the pupil enlarges to let in more light. In a bright room, says Loewenstein, a pupil is about 1 millimeter in size; in darkness, it can be 8 millimeters across.
The second mechanism involves cells in the back of the retina known as rods and cones. Cones are cone-shaped cells that are sensitive to bright light and color; rods are rod-like cells that are sensitive to dim light.
In order to work, the rods use a purplish pigment called rhodopsin. This pigment is bleached by light in "the same way light bleaches the color out of fabric," says Loewenstein. Before we can see well again in darkness, our eyes must regenerate more rhodopsin, and this requires some time.
There is a much shorter delay in adjusting to bright light because pigments have not been bleached and do not need to be regenerated.
Until recently, scientists believed the loss of rhodopsin was the major reason for the time needed to adapt to darkness. But recent experiments show that less rhodopsin is bleached by light than was believed, suggesting that other mechanisms may be at work as well.