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Refraction is a refraction of light. It also happens in the human eye, when light hits it and it is focused in such a way as to hit the retina and give a clear image of what we are looking at. When the eye’s refractive mechanism is disturbed, it becomes a visual defect. Find out how eye refraction works and what its defects are.
Eye refraction, or why we can see
The eye, in its schematic structure, does not differ much from a magnifying glass or binoculars. Light first enters the eye through the cornea, the transparent outer layer of the eye. The rays are then focused by a lens deep in the eye. Correct vision is the result of the light rays crossing precisely on the retina that lines the back of the eye. The retina is the photosensitive layer of nerve cells that converts light into nerve signals, making us see.
When we use binoculars or a microscope, we need to focus using the knob. The eye also has a similar mechanism of contraction of the ciliary muscle, which changes the shape of the lens and thus sets the correct light refraction angle depending on the distance between the eye and the object being viewed. This mechanism is called eye accommodation, that is, adjusting to the distance to obtain a sharp image.
Refraction and sight defects
Eye refraction and accommodation are the basic mechanisms responsible for correct, sharp vision. When any of them malfunction, it causes a visual impairment. The most important eye defects related to refraction and eye accommodation are:
- Farsightedness, commonly known as farsightedness, is a visual defect in which the retina is too close to the lens and the cornea, so that the light rays do not intersect and the image is not sharp.
- Nearsightedness is when the eye is not properly constructed and the light rays converge in front of the retina. As a result, a sharp image is created in a place where there are no photosensitive cells, and a blurred image reaches the retina.
- Astigmatism, or incoherence, is a sight defect in which the rays are refracted vertically differently than horizontally due to the disturbed symmetry of the eye. As a result, the image is blurred to one side and vision distortions such as straight line deformations.
- Presbyopia is the age-related decline in the ability of the lens to accommodate due to a loss of flexibility. As a result, the ability to see up close, read or see small objects is impaired.
Eye refraction test
The most basic method of eye examination is to use characteristic charts with smaller and smaller letters. The patient watches one of them from a distance of about 6 m, and the other, smaller one, is kept between 30 and 40 cm from the eyes. The smaller letters we can see without correction, the greater the visual acuity. In the event of a defect, this method is used again to select the appropriate corrective lenses. Modern ophthalmologic offices use an autorefractometer, a special computer that examines the parameters of the eye by computer, such as the size of the spherical defect, as well as the size and angle of astigmatism.
Treatment of eye refractive errors
The most common method of counteracting refractive errors is to correct them with glasses, thanks to which the light starts to focus properly on the retina. Short-sightedness is corrected with diffusing lenses, and farsightedness with focused ones. If astigmatism is also present, then toric lenses of irregular thickness are needed to correct the disturbed symmetry of the eye. With presbyopia, reading glasses are used to increase visual comfort in critical situations.
Contact lenses are an increasingly popular alternative to glasses. They require special attention while maintaining the rules of wearing hygiene, but they are more comfortable in everyday use.
There are also surgical methods for permanent correction of refractive errors. Usually, these are laser treatments that shape the cornea. In other cases, the shape of the cornea is changed with small incisions or a corrective lens is implanted into the eyeball. Each such interference requires detailed diagnostics, because not all vision defects can be eliminated in this way. It also depends on the specific case whether the refractive error of the eye will be completely eliminated or only reduced so that less powerful glasses are needed.
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