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How does the eye work?

Last Update: January 7, 2015; Next update: 2018.

Sight is our most complex sense. For our brain to be able to create an image of our surroundings, the eye needs to convert light into electrical signals called nerve impulses. These nerve impulses then travel to the brain along the optic nerve. The different parts of the eye have specific roles. These are the main parts:

Illustration: The main parts of the human eye

The main parts of the human eye

The colored part of the eye is called the iris. The iris is made up of muscle tissue and can expand and contract. That is how it controls 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.

The space between the cornea and lens is filled with a fluid called aqueous humor. It cleanses the eye and supplies the cornea and lens with nutrients.

The inside of the eye

Once light has passed through the pupil it reaches the lens. The lens then focuses the light, similar to the lens in a camera. The lens is attached to fibers and muscles that can pull on the lens to change its shape. That enables the eye to adapt to things that are closer up and further away – just like cameras and binoculars do. By adapting like this, the lens focuses light onto the retina in a way that helps us see images clearly. This process is called “accommodation.”

The back of the inside of the eye is lined with a membrane called the retina. The retina has more than 120 million sensory cells in it, which convert the light into nerve signals. The retina is like a digital camera chip that captures the image as many small points (pixels).

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 see in color.

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 part of the retina helps us to see clearly in bright light and is the central area of sharpest vision.

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.

Sources

  • Schmidt R, Lang F, Heckmann M. Physiologie des Menschen: mit Pathophysiologie. Heidelberg: Springer. 2011.
  • Menche N. Biologie Anatomie Physiologie: mit www.pflegeheute.de – Zugang. München: Urban & Fischer/ Elsevier GmbH. 2012.
  • Pschyrembel W. Klinisches Wörterbuch. Berlin: De Gruyter. 2014.
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    Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. We do not offer individual consultations.

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