The eye is a small organ that is present in most organisms. Eyes in the simplest organisms can detect whether their surroundings are light or dark, while complex eyes can distinguish colors and shapes. The human eye is a complex eye and specifically focuses on light because it is a camera-type eye. A camera-type eye is one that has a retina, which is a membrane that is sensitive to light and lines the surface of the eye. It is through the retina that the optic nerves form images.

The human eye contains multiple parts that all work together to allow a person to see images. The way the eye focuses on light rays can be thought of as a camera. The cornea is the outermost layer of the eye. It is used to focus incoming light. Behind the cornea is iris. The iris contains the pupil, which can expand or contract the opening found on the iris to control the amount of light that enters the eye. Behind the pupil is the lens. Important muscles known as the ciliary muscles surround the lens. Depending on whether or not the muscles contract or relax allows a person to see objects that are either far away or up close. When the ciliary muscles contract, it allows a person to see objects that are close up, but when the muscles relax, a person can see objects that are far away. Once light goes through the lens, it passes through the vitreous humor and then hits the fundus.  The fundus is a three-part tissue that contains the sclera, choroid, and the retina. The sclera is the connective tissue that gives the eye its white color. The choroid layer contains the blood vessels that supply nutrients and oxygen to the retina, as well as takes away the waste from the retina. The retina contains millions of light sensitive cells called photoreceptors.  There are two kinds of photoreceptors, rods and cones.  Rods are distributed throughout the retina, they are responsible for motion detection and form recognition, as well as vision under dim light environment.  They react equally to light of different colors.  Cones are mostly confined to a small area called the fovea centralis.  which measures 1.5 mm in diameter.   There are approximately 150,000 cone receptors per square millimeter in the fovea centralis and their density rapidly declines with the distance from the center.  It is the cone that processes information about color.  Cones are composed of  three types of pigment that selectively responds to different quanta of energy corresponding to red, blue, and green lights.  The rod and cone receptors contain molecules of pigment that absorbs visible light.  The photopigment molecules of the rod receptors are called rhodopsin. Rhodopsin is composed of two parts, the opsin and retinal.  When photons from the light rays  strike the rhodopsin molecules, the energy  excites the retinal and changes its chemical arrangement, from a cis (same side) to a trans (opposite side).  It is this cis-trans isomerization of the retinal that excites the photoreceptors and generates an electrical signal to the ganglion cells, which transmits the electrical signals to the brain and gives us the sense of sight.  Rods and cones work together to allow a person to see details and to see objects clearly.

Not only is the eye an important factor when seeing objects, but the brain also plays an important role in vision. When one looks at an image, photoreceptor cells examine all aspects of the image, noting color, shape, and motion. The cells send electrical impulses to the cortex of brain, where the impulses are analyzed and interpreted. The cortex forms the image and one is able to perceive the image. This process occurs everytime people look at images and takes about a quadrillionth of a second. An experiment was performed by John Hopkins Hospital in 1958 to discover how the cells in the visual cortex worked. Once the researchers had located the primary place in the cortex that vision takes place, a microscopic piece of the cortex was examined under a microscope. It was discovered that there were millions of cells in just the microscopic piece of the cortex. Researchers then went further in their new discovery and studied what the millions of cells actually did. It was discovered that the cells only reacted to certain lines of light at a specific angle. If the angle of the line of light changed by only twenty degrees, new cells in the cortex would start working, and the other cells would stop working. This experiment showed scientists and researchers how complex the cortex in the brain is when dealing with vision.