When you look at an object, light rays reflected from the object hit the transparent cornea at the front of your eye. The rays are partly focused and pass through the pupil, which enlarges or constricts depending on light conditions. The lens varies its focusing power for near and distant objects and fine-focuses the rays to create a sharp image on the fovea, the most responsive area of the light-sensitive retina at the back of the eye.
The mechanism of vision
Light rays focused by the cornea and lens produce an image on the retina that is upside down. Electrical signals from stimulated cells in the retina travel along the optic nerve to the brain, where the image is interpreted as being upright.
Action of the pupil
In dim conditions, the pupil widens (dilates) to allow the maximum amount of light to reach the light-sensitive retina. In bright light, the pupil constricts. Two sets of muscles in the coloured iris control these processes.
The eye adjusts for near and distant vision by changing the shape of its lens. This varies the extent to which incoming light is refracted (bent). To create a sharp image on the retina, light rays from near objects must be bent more than those from distant objects. This process is called accommodation.
Electrical signals from each retina pass along the optic nerves, which meet at a junction called the optic chiasm. Here, half of the nerve fibres from the left eye cross to the right side and vice versa, and the fibres continue along the optic tracts to the brain. Information from the right half of each retina passes to the right visual cortex; information from the left half of each retina goes to the left visual cortex. The brain then integrates these messages into a complete visual picture.
Rods and cones
There are two types of light-sensitive cells in the retina: rods and cones. Up to 120 million rods are distributed throughout the retina. Although rods are sensitive to all visible light, they contain only one type of pigment and cannot distinguish colours. They are therefore responsible mainly for night vision. In contrast, the 6.5 million cones provide detailed and colour vision. Every cone responds to red, green, or blue light, working only in bright light. They are most concentrated in the central part of the retina, the fovea.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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