Normally, light is focused by the cornea and lens to form a sharp image on the retina. Sometimes the eyeball is too long for the shape of the cornea, so the light comes to a focus before it reaches the retina.
As a result, objects in the distance are blurred although close objects are usually clear. Short- sightedness can be corrected by spectacles or contact lenses.
These lenses correct the vision so that distant objects are clear once again.