Eye

The eye is the organ of the body that receives light and converts it into sensory signals sent to the brain. The eye enables living things to perceive color, shape, movement, and distance.

Eyes vary widely among different species. Some animals have only simple light-sensitive tissue, while others have compound eyes like insects, or camera-like eyes like humans and other mammals.

Structure of the human eye

The human eye has different layers and functional parts. The cornea (transparent front layer) and lens (internal lens) focus light together. The iris (the colored part) controls the size of the pupil, which determines how much light enters. The retina is the back layer containing light-sensitive cells where signals begin.

The retinal cells are divided into rods, which function in low light, and cones, which distinguish color. These signals travel via the optic nerve to the back of the brain, where the image is interpreted.

How vision works

Light reflected from objects enters the eye, and the cornea and lens focus it onto the retina. The image that falls on the retina is inverted, but the brain corrects it. The internal lens changes shape to focus on objects both near and far; this process is called accommodation.

Vision disorders

The most common vision errors are myopia (nearsightedness), hyperopia (farsightedness), and astigmatism (corneal irregularity); these are usually corrected with lenses or contact lenses. Other well-known conditions include cataracts (clouding of the lens), glaucoma (pressure in the eye), and retinal damage related to diabetes.