A near-sighted eye is usually longer than a normal-sighted eye. Incoming rays of light are bundled so that their focal point is not on, but in front of, the retina. Distant objects are perceived in a blurred manner, whilst objects up close are in focus. The longer the eye, the more pronounced the degree of near-sightedness.
The interactive animation to the side of this text allows you to see how the eye and image perception change with varying degrees of near-sightedness.
The power of refraction of the eyes can be reduced surgically by means of laser correction or intraocular lenses, which shifts the focal point backwards onto the retina. In the case of glasses or contact lenses, this occurs by means of a concave lens, the strength of which is expressed in minus dioptres.