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The goal of LASIK is to either flatten or steepen the cornea uniformly so that the cornea now focuses
all the rays of light from an object being viewed on the retina at a sharp point. This results in the
object appearing to you as sharp and clear. This is done with an excimer laser which can very precisely
vaporize or remove minute amounts of corneal tissue.
Nearsightedness
As previously explained, in a nearsighted eye, the rays of light are bent inward by the cornea and brought
together at a point too soon, in front of the retina rather than on it. It has been experimentally
determined that the steeper the cornea is centrally, the more the rays of light will be bent inward.
Therefore if we don't want them bent in as much, we will need to decrease the steepness, or flatten the
central cornea. The excimer laser does this by removing corneal tissue centrally.
Farsightedness
Conversely, in a farsighted eye, the rays of light aren't bent inward enough. Therefore to increase how
much they are bent inward, we must steepen the central cornea. This is done by removing corneal tissue
peripherally with the laser which causes the central cornea to bulge outward and thereby become steeper.
Astigmatism
In astigmatism, only parts of the cornea need to be flattened rather than the entire central area. The excimer
laser is used to vaporize tissue just in these steep areas thus achieving this goal.
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