In refractive errors the
image of the object a person is looking at is not focussed properly onto the
retina (the light-sensitive tissue in the back of the eye).
For perfectly clear
vision, the image of a viewed object needs to be focussed onto the retina, just
as a camera has to be focussed properly in order to take a clear picture. If
the image is not focussed exactly on the retina, then the image will be
blurred, just like an out-of-focus photograph. In this case, the person is said
to have a refractive error.
Refractive errors occur
when there is a mismatch between the length of the eye and its optical power.
These mismatches usually originate during childhood when the eyes are growing.
The exact causes of
refractive errors are still being studied, but it is known that both hereditary
and environmental influences can affect their development. Most people have
some refractive error, but in most cases the error is small, and does not cause
any problems. In fact, the average person is slightly longsighted.
Refractive errors can
usually be corrected using spectacles or contact lenses. There are also
surgical techniques which can be used to correct refractive errors.