In people suffering from myopia the eyes keep growing and become too long. This means that the light entering the eye does not in fact arrive on the retina at the back of the eye. Instead the rays of light are focussed in front of the retina which leads to blurry distance vision. In some instances the cornea is also responsible for short-sightedness – it may be more curved than usual leading to refractive error and hence short-sightedness.
The precise reason for myopia is unknown but genetics and other factors that can interfere with the normal development of the eye are believed to play a role. For example if both parents are short-sighted there is a risk of one in two that the child will be myopic. There are 26 genes associated with short sight and as such it can be assumed that genetics can play a role in this condition.
Over recent years there has been much discussion about the effect of close work and use of mobile or tablet devices and their possible role in myopia. However none of these studies have provided conclusive data. It is always worthwhile taking screen breaks to rest your eyes from too much strain.