The eye is shaped like a sphere, with the majority of it resting in the eye socket, protected by the bones of the skull. Your eyelid protects the exposed portion of your eye, the part that allows sunlight to enter and enables you to see.
Epithelium & Sclera
The sclera is the white part of the eye and the outermost layer. It is a thick membrane made of tough fibrous tissue that protects the eye from debris, dust and other possible unwelcome intruders. The epithelium is a thin layer below the sclera. It also protects the eye, mainly the cornea, from scratches and scrapes and has the ability to heal quickly.
Underneath the epithelium is the cornea, which is thicker than the epithelium, and a curved band of strong clear tissue. Also known as the window of the eye, this is where outside light first passes through the eye. When light hits the eye, the cornea focuses the rays onto the retina, the membrane responsible for converting light to images that are sent to your brain.
Found below the cornea, the iris is the colored part of your eye -- usually blue, brown, green or hazel. While the color is inherited genetically and doesn't affect sight, it is often a sign of beauty and source of attraction. As a muscle, the iris' function is to control the size of the pupil, and therefore the amount of light that passes through the eye.
When you look in someone's eyes, the small, black dot you see in the center is the pupil. Once light enters the eye through the cornea, the pupil controls the amount of light that reaches the retina. When more light is allowed to pass through the pupil, the iris expands and the pupil appears bigger; when less light is allowed to pass through, the iris contracts and the pupil appears smaller.
The lens is located directly behind the pupil and is held in place by the ciliary muscle, which changes its shape as the eye focuses. The natural structure of the lens is similar to a magnifying glass; its sole purpose it to focus these rays of light into the retina. People need glasses when a lens is weak, or doesn't function as well as it should.
The retina is a membrane on the back wall of the eye that works very similarly to camera film and is made up of millions of tiny light receptors. When a ray of light passes through the cornea and the pupil, it hits the retina which transforms that light into pictures that are sent to the brain through the optic nerve.
Rods & Cones
One of the layers inside the retina contains cells responsible for how the transmitted light is interpreted by the optic nerve. The cells are either cones or rods, named after their shape. There are an average of 75 to 100 rods and 7 million cones in the retina. Rods interpret light and dark tones in an image; cones interpret the fine lines and points. The 7 million cones are divided into three types; the first absorbs blue-violet wavelengths, the second absorbs green wavelengths, and the third absorbs yellow and red wavelengths.