It is the medical evaluation of ocular pathologies, assessment of the fundus, the measurement of the eye blood pressure, treatment of diseases such as conjunctivitis, cataracts and pterygium.
People who need to wear glasses in order to see clearly are carriers of the so-called "refractive errors", known as myopia, hyperopia and astigmatism, among the main ones. These refractive errors, which are mostly attributable to defects in the cornea (transparent lens that is on the outside of the eye and through which the iris is observed which is the green, blue or brown part of the eye) cause the images not to be projected onto the retina with clarity.
The eye could be compared to a photographic camera, if the lens is out of focus the images projected on the film are blurred. The same thing happens if there are refractive errors, the images projected on the retina are blurred.