The human eye can move in many directions to maximize the field of vision. The lens, which is used to fine-tune vision, can change shape because it is attached to other muscles. Some move the eye towards the nose, or away from the nose. Others raise, lower or rotate the eye. The iris -- the colored part of the eye containing the pupil -- also has muscles that enable it to expand the pupil to allow more light into the eye, or contract to allow less light into the eye. By changing the size of the pupil, the eye can adapt to different light levels in the environment.
While reading in low light will not damage your eyes, most scientists agree it can cause eye strain, which can lead to discomfort and possible long-term effects. As you read, the iris, as well as the muscles that control the shape of the lens of your eye, must contract to keep the focused image on the retina. In low light, the visual muscles receive mixed signals. On the one hand, they need to relax to collect as much light as possible to compensate for the poor lighting. But they also must contract more to maintain the focused image, because in poor light, the contrast between words and page is not as great. Your eyes have to work harder, and can become strained. Common symptoms of eye strain include sore eyeballs, headaches, back and neck aches, drooping eyelids, and blurred vision. Prolonged eye strain can make near-sightedness worse.
Blindness occurs when light-collecting cells in the eye's retina are damaged. So scientists are developing artificial retinas to restore at least some sight to the blind. One alternative is an implanted silicon microchip, which converts light into electrical pulses just like the damaged cells. Another is to make artificial retinas out of light-sensitive cells in spinach leaves, which do much the same thing for plants during photosynthesis.