The retina is made up of millions of light-sensitive nerve cells called photoreceptors. Photoreceptors contain special chemicals which change when light hits them. This change causes an electrical signal which is sent to the brain via the optic nerve. Different types of photoreceptor allow us to see in a huge range of different conditions, from dark to light, and all the colours of the rainbow.
There are two kinds of photoreceptors: rods are very sensitive and help us to see in dim light. They are also very sensitive to movements, particularly at the edge of our vision - but they are not sensitive to colour. For this reason colour perception is partly lost when there is little light. Cones give us colour vision; they function best in bright light. Cones are most concentrated in our area of central vision.
The electrical signals from the photoreceptors travel to a part of the brain called the thalamus via the optic nerve. This area acts as a relay station, combining information from the two eyes and sending on the information received to an area of brain called the visual cortex. The visual cortex is a specialised part of the brain which processes visual information. Located at the back of the head, it interprets the electrical signals to obtain information about the object's colour, shape and depth.