Eyes are organs belonging to the visual system. They provide vision to animals, this refers to the ability to receive as well as process visual detail. Eyes detect light then convert it into impulses (electro-chemical) in neurons. In higher organisms, this organ is a complex optical system that collects light from the environment, regulates its intensity via the diaphragm, focuses the light with the help of an adjustable assembly of lenses in order to form an image. This image is then converted into electrical signals and transmitted to the brain by complex neural pathways connecting the eye through the optic nerve to the visual cortex as well as other areas of the brain.
Simple eyes, like those of microorganisms, do not detect much. They only detect if the environment is dark or light.
Complex eyes on the other hand can differentiate colors and shapes. Many organisms have large visual fields especially predators, they have a binocular vision in order to improve their depth perception. In some organisms, the location of the eyes affects the field of view. For example, the location of the eyes of horses and rabbits is to maximize the field of view. These animals have a monocular vision.
Compound eyes are found in arthropods and they are made up of numerous simple facets which may give either multiple images or a single pixelated image per eye, depending on the anatomy. Every sensor has got its own photosensitive cells and lens. Some eyes have approximately 28,000 such sensors that are arranged hexagonally, enabling a 360⁰ field of vision. These eyes are very sensitive to motion. With every eye viewing something different, a fused image from both eyes is produced in the brain producing different and high-resolution images.
Simple eyes are those eyes that having a single lens. For example, jumping spiders are known to have a pair of simple eyes having a narrow field of view. This is supported by other smaller eyes for purposes of peripheral vision. The simplest eyes are known as ocelli and they can be found in animals such as snails that do not actually see. These eyes have photosensitive cells but they lack lens and other means of projecting an image onto cells. They can only distinguish between dark and light.
