The Brain and Eye
When light is reflected from an object, it carries more information than the brightness of the object. It also carries information about color, depth and motion. An eye that can gather this information is usually attached to a brain that can make sense of it.
The interaction between the eye and brain is almost instantaneous and constant. Light enters the eye, the eye transmits signals to the brain, the brain sorts and assembles the signals into images.
Eyes that gather a variety of information from light can be divided into two groups: single-lens eyes and compound eyes. These eyes have very different structures, even though their basic function is the same -- to gather information for an image-making brain.