Light comes in through the cornea, which focuses the light rays as they pass through the pupil and the lens. The iris, which surrounds the pupil, works like a camera shutter, controlling the amount of light that enters. In normal eyes, the cornea and lens bend light so that it focuses directly on the retina, the light sensitive tissue in the back of the eye. The retina changes light into electrical impulses that travel via the optic nerve to the brain. Poor vision results when the cornea and the lens focus light too far in front of or behind the retina.
Corrective lenses work by causing the light to focus directly on the retina.
When someone is nearsighted, light entering the eye is focused in front of the retina, instead of directly on it. People with myopia see near objects more clearly, while far objects appear blurry and out focus. Myopia is an inherited condition, which usually starts in childhood and stabilizes in the late teens or early twenties. Astigmatism is the eye condition that occurs when the surface of the cornea is uneven, causing light to fall unevenly on the retina. The incisions are used to flatten the surface of the cornea to reduce nearsightedness.