According to their purpose glasses are corrective, protective, colored and special.
Corrective glasses are prescribed to correct certain optical problems of the eye-myopia, high hyperopia, astigmatism, weakening of accommodation. Human eye, regarded as an optical device, can be compared to a sophisticated photographic apparatus. The eye has also a system for focusing the rays of light and a screen on which images of the surrounding objects are projected. In many cases the optical system of the eye is properly read and images of the objects are just focused on the retina. These eyes do not need glasses. In other cases, the focus of the optical system is incorrect. There are three types of abnormal focus-myopia, hyperopia and astigmatism. The eye has no ability to correct nearsightedness and astigmatism. This may be done by using glasses that correct what is not enough for the proper focusing of images on the retina.
In farsightedness things are a bit different. Thanks to the ability to amplify the reflecting force of the lens (the so-called accommodation) small degrees of hyperopia are covered by the accommodation reserves of the farsighted at a young age and they see both far and near. But when they get older the accommodation ability decreases. That’s why people with great hyperopia are forced to put glasses at a young age. Very often they have two pairs of glasses-for looking at a short distance and for looking at far distance.
People with normal refraction are forced to put glasses at the age of 45 due to physiological weakening of the accommodation abilities. People with astigmatism have difficulties in seeing both far and near. They definitely need to wear glasses constantly.
The issue of prescription of glasses to children is more complex. Usually, the majority of new-born and young children are farsighted. With the growth of children farsightedness may be reduced or even turn into myopia. The reverse process-turning from nearsighted to farsighted is impossible. In many children with high hyperopia occurs strabismus. This most often occurs between 1 and 3 years old. If such a child wears it glasses constantly the strabismus may disappear. That’s why every little child who has strabismus is examined for hyperopia. This examination has the aim to provide timely treatment. A child with strabismus can be treated successfully only if parents are sufficiently insistent on execution of medical prescriptions.
Protective glasses are used when working in industries where there is danger of fall of foreign bodies in the eye.
Colored glasses are prescribed to patients whose eyes are sensitive from the sunlight.
Specialized glasses are used by people with poor sight. These glasses increase the image and are used by engravers, watchmakers, beauticians, surgeons and others. Welders also wear such glasses in order to protect their eyes from UV radiation.