In the normal eye, the cornea focuses light rays directly on the retina, resulting in clear vision without glasses or contact lenses.
In myopia, or short-sightedness, the eye is longer than normal. In some cases, the cornea may be more curved than normal.
The light rays go through the cornea but come together at a point in front of the retina, and are out of focus on the retina.
In short-sightedness, distant objects are blurred, while nearby objects can be clear.
You might be surprised that genetics is not the only factor in short-sightedness. Environmental factors also play a role. For example, short-sightedness almost doesn’t exist among members of traditional societies. As soon as you send children to school, however, about 40% of them develop this refractive error. It appears that there might be cells in the retina that respond to environmental demands of modern societies.