In the normal eye, light enters through the cornea (the clear front surface of the eye) and comes to a focus on the retina in the back of the eye. 

Myopia, or nearsightedness, occurs when the eye is longer than average or the cornea's curvature is steeper than average. This causes the light to focus somewhere in front of the retina, resulting in blurred distance vision. People who have myopia can usually see well up close. This type of refractive error usually develops at an early age.

Myopia is usually corrected with glasses or contact lenses, but it can also be corrected surgically with LASIK or PRK.