The excimer laser emits a cool beam of ultraviolet light of a specific wavelength (typically 193 nanometers) to precisely remove corneal tissue. When the surface of the cornea is reshaped in the right way, it allows light rays to focus properly onto the retina for clear vision.
The high-energy pulses of ultraviolet light penetrate only a tiny amount of the cornea and have the ability to remove as little as 0.25 microns of tissue at a time. (One micron is a thousandth of a millimeter.)
An excimer laser corrects nearsightedness by flattening the cornea; it corrects farsightedness by making the cornea steeper. And astigmatism can be corrected by smoothing an irregular cornea into a more symmetrical shape.