When you understand how the eye works, it is easier to comprehend why people need glasses or contact lenses to see well.
The eyes are complex sensory organs. They are designed to optimize vision under conditions of varying lighting. The basic elements are similar to those of an average photography camera.
The primary function of the eye is to focus light. For the eye to see, light rays must be bent or "refracted" to meet at a single point through the cornea, the clear window at the front of the eye that provides most of the focusing power. The transparent dome cornea is a firm covering and is susceptible to dryness and abrasion injuries.
The iris of the eye is the color portion behind the cornea. Our eye color is a function of the amount of pigment within the iris (brown eyes have the most pigment, while blue eyes have the least). The iris contains muscles that open and close its central opening called the pupil in response to decreases and increases in light exposure (exactly like the camera aperture).
Light then travels through the lens, where it is fine-tuned to focus properly on the retina, the nerve layer that lines the back of the eye and connects to the brain. The retina acts like the film in a camera, and clear vision is achieved only if light from an object is precisely focused onto it. If the light focuses either in front of or behind the retina, the image you see is blurred. A refractive error means that the shape of eye structures does not properly bend the light for focusing.
Having 20/20 vision means seeing at 20 feet what a normal person sees at 20 feet. However, if vision is measured at 20/40, it means a person has to walk up to 20 feet to see the same size letter that someone with 20/20 vision could see at 40 feet. People whose best- corrected visual acuity (what they see using glasses or contact lenses) is less than 20/200 in the better eye are considered legally blind, even though they still have enough vision to get around.
What are the whites of the eyes?
The white outer layer of the eyeball is called the sclera. The sclera is covered by a fine tissue containing tiny blood vessels called the conjunctiva. (It is the dilation and proliferation of these blood vessels that accounts for the pinkness of conjunctivitis or "pinkeye.")
What is the middle of the eyeball made of?
The central core of the eyeball is a gel-filled area called the vitreous. Behind the vitreous, at the back of the eye is the retina. The retina is composed of 10 fine layers of specialized nervous tissues that respond to light exposure by initiating chemical reactions that are translated into electrical signals which are then transmitted to the back of the brain through a large nerve in the back of the eye (optic nerve).
What muscles move the eyeballs?
Six different muscles located around the eyeball are capable of directing the eyes toward the subject to be seen.
What are tears?
The eyes are protected by the tears normally produced by the tear glands (lacrimal glands) located above the outer portion of each eye. The tears drain from the eyes through the tear ducts, located on the inner portion of the eyes. These ducts drain into the nose. This is why our noses run when we weep!
For more in-depth information about eye care, please read the MedicineNet.com Eye Care article.