The average eyeball is small — approximately an inch wide, an inch deep and 0.9 inches tall (slightly smaller than a gumball). But this globe-like structure is a complex, finely-tuned, pretty amazing mechanism, with an array of parts that work together to allow us to see.
The process behind our vision:
"The eye is a container,” says Richard Rosen, M.D., a vitreoretinal surgeon at the New York Eye and Ear Infirmary of Mount Sinai, in New York City. The outside layer of the eye is a tough white protective layer called the sclera (more commonly known as the “white of the eye"). “Composed of collagen fibers, which give it a dense structure, the sclera provides some protection for the eyeball,” says Rosen.
Light reflects off the object we're looking at and enters the eye through the cornea, a clear, thin, dome-shaped tissue at the very front of the eye. The cornea has a curvature to it and covers the eye, kind of like a crystal covering the face of a watch. “When rays of light enter the eye, they're sort of parallel to each other,” says Rosen. “But as they pass through the cornea, they bend and start to converge, almost coming to a point on the retina.” From there, light travels through a clear fluid, called the aqueous humor, which fills small chambers behind the cornea, nourishes the eye and helps retain pressure to help the eye retain its shape.
As the light continues, it passes through an opening called the pupil , that black dot at the center of the eye. The pupil is surrounded by the iris, the colored part of the eye. It's the iris's job to control how much light the pupil lets into the eye. When there is bright light, the iris uses muscles to change the size of the pupil (making it contract) to let in less light. When there is low light, the iris opens up the pupil, making it wider, to let in more light.