The human eye

The eye is probably the best optical instrument we have, although it only has one convex lens. The power or focal length of this lens can be controlled by special muscles, called cilliary muscale. The lens is used to focus the light rays on to the back of the eye, or retina. This sends a message through the optic nerve to the brain

When you look at a distant object your eye muscles make the lens thinner. The light ray from the distant object are then focused on the retina. When you look at an object close to you eye, the muscles make the lens fatter, or more convex, so the object is focused.

The amount of light enetering the eye is controlled by the pupil. This is an opening in front of the eye lens which can be made larger or smaller depending on the brightness of the objects we are looking at. At night the pupils become wide open, to let as much light in as possible. But on a bright day the pupil shrinks to avoid straining the eye. This works much in the same way as the camera lens.