The human eye belongs to a general group of eyes found in nature called “simple eyes“. It contains a single lens that focuses light onto a light-sensitive membrane called the retina. Light-sensitive cells on retina convert light into electro-chemical signals, which are sent to brain via optic neurons to form the images that we see.
Behind the aqueous humor is a colored, ring-shaped membrane called the iris. It has an adjustable opening called the pupil (circular in human eyes, but may take different shapes in other animals), which can expand or contract to control the amount of light entering the eye. The pupil appears black because most light entering the eye through it gets absorbed and doesn’t come out. The iris contains two groups of smooth muscles; a circular group called the sphincter pupillae, and a radial group called the dilator pupillae. Contraction of the sphincter pupillae reduces the size of the pupil, while contraction of the dilator pupillae dilates the pupil.
Situated behind the pupil is a transparent structure called the crystalline lens. Ciliary muscles surround the lens and hold it in place. Furthermore, when the ciliary muscles relax, they pull on and flatten the lens, allowing the eyes to see objects that are far away. To see closer objects clearly, the ciliary muscle must contract in order to thicken the lens (this is why when our eyes get tired after reading a book for a long time). The interior vitreous chamber of the eyeball is filled with a jelly-like tissue called the vitreous humor. After passing through the lens, light must travel through this humor before striking the light-sensitive layer of cells called the retina, on which images are formed.
Embedded in the retina are millions of light sensitive cells, which come in two main varieties: rods and cones. Rods are used for monochrome vision in poor light, while cones are used for color and for the detection of fine detail. Cones are packed into a part of the retina directly behind the retina called the fovea, which is responsible for sharp central vision. When light strikes either the rods or the cones of the retina, it’s converted into an electric signal that is relayed to the brain via the optic nerve. The brain then translates the electric signals into the images that we see.