Eye types range from simple to the complex and the compound. Eyes are organs that detect light. Simple eyes detect whether it is light or dark; while complex eyes can detect color, motion, depth, height, width, length and distance. Compound eyes, found in insects, crustaceans, arachnids and other multi-jointed legged creatures, combine what they see into a single image or multiple images.
The human eye is a white, nearly spherical piece of tissue with a colored circle, called an iris, and an opening in the center, called a pupil, in the center. It connects to the brain by an electrical conduit called the optic nerve. All vertebrates and some mollusks have a similar eye structure. Arthropods (insects, spiders, scorpions, crustaceans, centipedes and millipedes) have what is called a compound eye. Compound eyes have several thousand small eyes, which form a sphere around the head of the arthropod allowing them to see in a 360-degree circle around themselves. Each eye may see either pieces of the whole or multiple images of the whole, depending on the species.
The human eye is a bulging sphere, meaning that it is not as tall as it is wide. It contains the cornea, which allows light to enter the eye and bends it, helping to focus the image. The iris surrounds the pupil and is the eye color of the individual. The pupil expands and contracts, allowing a specific amount of light to enter the eye. The lens bends to focus near or distant objects. The retina has color-sensing cells called rods and cones. The retina also has neurons that help process the information received by the rods and cones. The optic nerve takes all the information and sends it to the brain. Each compound eye contains a lens, a cone, light-sensitive visual cells and pigment cells, similar to a human iris, to distinguish each eye.
Human eyes are fixed in the center of the head such that each eye's field of view overlaps, allowing the ability to perceive depth and distance. Most mammalian, reptilian and avian predators have this type of eye position. Crocodiles, alligators and hippos have eyes positioned on top of the head, so that their bodies may remain submerged in water in order to hunt or in the case of hippos, to prevent sunburn. Other animals, like herd animals, whales, snakes, lizards and non-predatory birds have eyes on the sides of their heads, allowing them to see on either side and behind them, in order to remain in their herd or flock or to watch for predators. Insects, especially those that fly, have eyes situated on top of the head, allowing them to see in front, on both sides, behind and on top at the same time.
Human eye colors include variations of blue, green, blue-gray and brown. Animals have a wide range of eye colors. Eagles have yellow eyes, most wild mammals have brown eyes, and domestic dogs have a wide variety of eye colors, from light blue to yellow to brown. Insects like flies have red eyes. Tigers have yellow-orange eyes, if their coat is orange, but may have light-blue eyes if their coat is white. Gorilla eyes are black with light-brown irises.
Humans perceive light and their surroundings in the visual range. The visual range of light is divided into the colors red, orange, yellow, green, blue, indigo and violet. Other animals, like bees, see in the ultraviolet range. Scientists divide the ultraviolet range into three sections, near, far and extreme ultraviolet; near being the closest to visible light and far being closest to X-rays. Other animals, like rattlesnakes see in the infrared spectrum. Scientists divide infrared light into three sections, near, mid and far; near being closest to visible light and far being closest to microwaves. The infrared spectrum measures how much heat reflects from an object.