Nearsightedness: Causes of Myopia
In order for our eyes to be able to see, light rays must be bent or refracted by the tear film, the cornea and the lens so they can focus on the retina, the layer of light-sensitive cells lining the back of the eye. The retina receives the picture formed by these light rays and sends the image to the brain through the optic nerve, which is actually part of the brain.
Myopia occurs when the eye is longer than normal or has a cornea (clear front window of the eye) that is too steep. As a result, light rays focus in front of the retina instead of on it. This allows you to see near objects clearly, but distant objects will appear blurred.