The cornea is a  part of  the eye  that helps  focus light to create an  image on the retina. It works in much the same way that the lens of a camera focuses light to create an image on film. The bending and focusing of light is also known as refraction. Usually the shape of the cornea and the eye are not perfect and the image on the retina is out-of-focus (blurred) or distorted.

These imperfections in the focusing power of the eye are called refractive errors. There are three primary types of refractive errors: myopia, hyperopia and astigmatism. Persons with myopia, or nearsightedness, have more difficulty seeing distant objects as clearly as near objects. Persons with hyperopia, or farsightedness, have more difficulty seeing near objects as clearly as distant objects. Astigmatism is a distortion of the image on the retina caused by irregularities in the cornea or lens of the eye. Combinations of myopia and astigmatism or hyperopia and astigmatism are common.

In LASIK surgery, precise and controlled removal of corneal tissue by a specialized laser, reshapes the cornea changing its focusing power. LASIK is a procedure that permanently changes the shape of the cornea, the clear covering of the front of the eye, by using an excimer laser. A very precise instrument, called a microkeratome, is used to create a flap in the cornea. A hinge is left at one side of this flap. The flap is gently folded back revealing the stroma, the middle section of the cornea. Pulses from a computer-controlled laser vaporize a portion of the stroma and the flap is gently replaced.