How the eye works

A basic knowledge of how the eye works is essential to understanding the different causes of sight loss. Here is a basic outline in eight simple steps:

1. Light enters the front of the eye through the clear cornea and lens.
2. The iris, the coloured circle at the front of the eye, controls the amount of light that enters the eye.
3. The cornea and lens focus the light on to the retina at the back of the eye.
4. The middle of the eye is filled with a clear jelly-like substance called the vitreous.
5. The retina at the back of the eye is a light-sensitive layer which consists of rod and cone cells.
6. Cone cells are concentrated in the centre of the retina where the light is focused by the cornea and lens. This area is called the macula. Cone cells enable detailed vision and are also most responsible for colour vision.
7. Rod cells are concentrated around the edge of the retina. Rod cells enable vision in dim light, movement perception and general vision.
8. The rod and cone cells collect the light signals directed onto them and send them as electrical signals to the optic nerve at the back of the eye. The electrical signals are passed along to the brain where they are processed into images.

• Date: May 2009

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