How does the eye work?

Seeing is a complex process involving different parts of the eye:

• the iris
• the cornea
• the lens
• the retina
  
  
  

When you look into someone’s eyes, the first thing you usually notice is what color their eyes are. The colored part of the eye is called the iris. The iris is made up of muscle tissue and can expand and contract. This is how it can control how much light enters the eye. If it is very bright, the iris contracts so that we are not dazzled by the light. In the dark, the iris expands so that as much light as possible gets into the eye and we are able to see things around us. The round black hole in the center of the iris, where light enters the eye, is called the pupil.

The iris and pupil are covered by a transparent layer called the cornea. Its main function is to protect the eye from foreign objects and injury. Our eyelids, eyelashes and tear fluid do this too. But the cornea also plays a role in vision. It changes the path of the light (refracts the light) on its way into the eye.

Between the cornea and lens there is a fluid called aqueous humor. This fluid cleanses the eye and supplies the cornea and lens with nutrients.

Once light has passed through the pupil it reaches the lens. Similar to the way a camera does, the lens focuses the light. It is attached to fibers and muscles, which means that it can change its shape to make the light focus on the retina so you can see images clearly. This process, known as “accommodation”, makes it possible for us to see things that are far away as well as things that are close. Like a camera or binoculars, the eye has to focus.

The back of the inside of the eye is lined with a membrane called the retina. It has more than 120 million sensory cells in it, which convert the light into nerve signals. If we stick to the camera comparison: the retina is like photographic film. It is the part of the eye which captures the image.

There are two kinds of sensory cells, called rods and cones. Rods allow black-and-white vision at twilight and at night. Cones allow us to detect color. But these different kinds of sensory cells are not evenly spread out across the retina. The cones are most densely packed at the center of the retina, called the macula. This is the area of the retina which helps us to see clearly in bright light, and the area we focus our attention on.

The nerve signals that are sent by the sensory cells are carried along the optic nerve to the brain, where they are processed to form the images we see.

Author: German Institute for Quality and Efficiency in Health Care (IQWiG)

© IQWiG (Institute for Quality and Efficiency in Health Care) Last update of this website: 05/22/2012