Los defectos de refracción

When a wave like light passes through a medium incising in it obliquely a change of direction called refraction occurs, which can be: 

• Convergent: when the waves come together. 
• Divergent : when the waves separate when striking in the middle. 

The diopter is the unit that refers to the power of refraction, positive if it causes convergence and negative if it causes divergence.

Specialists in Ophthalmology establish that the light beams reach the retina and this transforms them into nerve signals that are sent to the brain, where the image that we see is formed. The eye has an optical system that makes all the light beams converge on the retina, so that the image looks sharp.

Composition of the eye

The refractive system of the eye is formed by two structures: the cornea, transparent external layer of the eye, and the lens, natural lens located behind the iris and the pupil. The light goes through these two structures until reaching the macula, area of ​​the retina with greater visual capacity.

The cornea contributes about 43 diopters to the eye, thus becoming the most powerful structure of the same. This power depends on its curvature, the higher it is, the more power it will have.

Thanks to the lens , the eye has 19 more diopters, but unlike the cornea, it can modify its power by adding up to 3 more diopters.

This phenomenon is called accommodation. Consists of the ability to, when looking at a nearby image, modify its shape, bulging and increasing its power depending on the proximity of what you want to see.

When we are looking at a distant image, the crystalline lens remains in its flattest position and with the least power. Accommodation is important for vision because it allows more convergence when focusing on nearby objects that arrive with more distant beams of light and we can, for example, read. When all the light beams of an object are in the same spot on the retina, the image we perceive will be clear. The problem is that the eye perceives light beams oblique and straight mixed together. The recti do not change when accessing the eye and focus on the same point of the retina. The obliques are modified by the refractive system of the eye, making them converge at the same point as the straight ones so that a clear image is produced. When the eye is not able to do this, a refractive error occurs.

Types of refractive errors 

• Myopia. It occurs when there is too much convergence in the ocular optic system, so that the oblique light beams converge at a point before the retina and when they reach it they do so separately, causing an out of focus image. Because of this, the myopic see badly from afar, since the near images need less convergence and can see them focused. The solution for myopia is the use of divergent lenses that compensate for the convergence of the eye, bringing the point of attachment of the light beams to the retina. 
• Farsightedness. It occurs when the eye can not get enough convergence of the light beams in the retina, joining said beams of light behind the retina. This causes an unfocused image. The solution for hyperopia is the use of converging lenses that compensate for the eye's deficit. 
• Tired or Presbyopia. It occurs when there is a lack of accommodation produced by age, usually after forty-two years. This progressively increases the difficulty to see up close in the person suffering from it. The tired or presbyopic sight is produced because the ciliary muscle loses strength due to fatigue and the increase in the hardness of the lens, so that it does not change shape and it is impossible to increase the diopters to see up close. The solution for presbyopia is the use of converging lenses that compensate for the eye's deficit in the fence. 
• Astigmatism. It occurs when the oblique light beams entering the eye are separated into two perpendicular planes with different convergence powers. This causes the half to focus on one plane and the other half on the other. The solution for astigmatism is the use of toric lenses that equal the convergence of all the light beams that enter the eye.