Under normal conditions, the light passing through the cornea and the crystalline lens will focus precisely on the retina. This is the ideal situation in which there is no refractive error and is called "Emmetropia". Therefore, emmetropia occurs when the refractive media of the eye (cornea and crystalline lens) and their distances from the retina are in complete harmony with each other. The cornea and the lens focus the light rays emitted from an object directly onto the retina, creating the right image. But vision is not perfect for all people. In some eyes the light is focused on the front and not on the retina. In other eyes the light focuses behind it. In refractive surgery, a specially trained ophthalmologist uses very precise lasers to gild the surface of the cornea and thus changes its curvature so that the light focuses directly on the retina.
Before undergoing refractie surgery, it is helpful to understand how the eye works. The eye is a simple sphere, about 2.5cm in diameter, yet its power is awesome. It discerns colors and shapes. In bright or dim light.
From near or far. It helps you read books, and situations, and people. It is a vital link to the world around you. And it succeeds through a complex ballet of muscles and nerves.
The cornea is the clear, dome-shaped window that forms the front wall of the eye and the retina is the light-sensitive tissue in the back of the eye that connects to the brain. Light reflected off an object passes through the cornea. Muscles in the eye contract or relax to adjust the shape of the lens, focusing the light rays.
The rays then reach the retina, where over 100 million light-sensitive cells read them and transmit the image through the optic nerve to the brain. Because the light rays cross while going through the cornea, the retina reads the image upside down—but the brain readjusts so you stay properly oriented.