Vision Basics
From Light To Image: The Physiology of Seeing
Human eyes are highly complex and specialized. In all types of eyes, sight is made possible by visual pigments which are found in photosensitive cells. These visual pigments convert light into electrical signals through a chemical change in the pigment. The electrical signal created by this chemical change is then sent to the brain through the optic nerves.
Anatomy of the human eye

Figure 1. Anatomy of the human eye. The cornea, lens, and vitreous are normally clear so that light can reach the retina unimpeded. The iris normally has color (brown, blue, hazel, etc.) while the opening in the center of the iris (pupil) allows light to pass through. When light hits the retina, an electrical signal is set up which is transmitted by the optic nerve through pathways to the visual part of the brain.

We are able to see a wide range of colors. Receptors in your eye that are responsive to color are called cone cells. Cone cells are located at the back of your eye in the retina. This is also where rod cells are found but they are responsible for vision in dark conditions, such as at night and are not sensitive to bright conditions. Cone cells are made up of different photopigments, which are sensitive to different wavelengths of light. Color vision works by mixing different lights to produce all the colors humans can recognize.

For more information on color vision, click here.

Human eyes can see things both up close and far away. In order for this to happen, the lens of our eye must change shape. Muscles in the eye accomplish this by either making the lens fat and round for seeing things up close or flattening and thinning the lens for seeing things far away. As we get older, especially after age 40, the lens does not change its shape so easily and people frequently have difficulty focusing up close, as for reading.

Figure 2. The visual pathway. Each eye represents two halves of the visual field. The left half of each eye represents the right visual field. Light hits the retina and passes through the retinal nerve cells, the optic nerve, the optic tract, and the lateral geniculate body to the visual portion of the brain, the visual cortex, where the image is actually "seen." Note that the projection of the visual field of each half of the eye is preserved in the brain.

Click here for information on amblyopia, binocularity, and monocularity.