What do we call a refractive agent?
A refractive medium is any transparent body that changes the angle or direction of the rays of a light beam that passes through it.
Refractive Abnormalities
What is the function of the eye as a visual system?
A beam of light rays, coming from a light source, enters the eye and reaches the retina. As the light beam travels towards the retina, it encounters the cornea, the aqueous humor, the crystalline lens, and the vitreous body. From theretina, through photoreceptors and special neurons, visual stimulus is transferred to the visual cortex of the brainto perceive the image.
What are the most refractive points of the eye?
The most refractive points during the course of the light beam are the cornea and especially its anterior surface and, to a lesser extent, the crystalline lens. The total refractive power of the eye is about +60 diopters (D). Of these, the cornea (anterior and posterior surfaces) offers about +43 to +47 diopters (D).
What is emmetropia?
Emmetropia is the refractive state of the eye, in which a parallel beam of rays from a distant light source is focused directly on the retina. In essence, the emmetropic eye has no refractive error.
What is ametropia?
In ametropia there is some degree of refractive error that prevents the parallel rays of light from being focused directly on the retina. The result is that the parallel ray of light is focused in front of or behind the retina, causing blurred vision.
What is myopia?
In the myopic eye, a parallel beam of light rays, coming from a distant light source, is not focused on the retina, but in front of it. This occurs either due to high refractive strength ordue to longer axial length of the eye. In some cases, both mechanisms may coexist. Consequently, the myopic eye is unable to distinguish distant objects. However, when the object gets closer to the myopic eye, its image will focus on the retina and the object will become visible with
Myopia: a parallel beam of light rays, coming from a distant light source, is focused in front of the retina.
What is hyperopia?
In the hyperopic eye, a parallel beam of light rays, coming from a distant light source, meets the retina before it even forms a focus. This occurs either due to low refractive power, or due to short axial length of the eye, or due to both mechanisms. The hyperopic eye cannot distinguish either distant objects or nearby ones. This is because the beam that sends the nearby object shifts the focus even further back from the retina.
Hyperopia: a parallel beam of light rays, coming from a distant light source, meets the retina before it even forms a focus.
What is astigmatism?
In astigmatism, light rays from both distant and nearby objects cannot be concentrated on a single point. This is due to differences in corneal and/or lens curvature in the different meridians. Astigmatism affects both distant and near vision and can coexist with any other refractive error.
What is accommodation
With the mechanism of accommodation, the eye changes its refractive power through a change in the shape of the crystalline lens. Through the contraction of the ciliary muscle, the zonular fibers, which are essentially the “support” of the crystalline lens, are relaxed. As a result, the crystalline lens becomes more spherical and its refractive power increases. Thanks to the mechanism of adaptation, the eye can clearly visualize the retina and the nearby objects.
What is presbyopia?
The main symptom of presbyopia is poor near vision. Over the years, the lens of the eye hardens and becomes difficult to change shape. Based on the mechanism of accommodation, it is understood that the range of accommodation decreases over the years. However, while the reduction in the range of accommodation begins in the early years of life, the effects and discomfort in everyday life are not seen until later, around the age of 40.
How are refractive disorders and presbyopia corrected?
The correction of refractive errors and the improvement of presbyopia are performed in the following ways:
- Corrective glasses
- Lenses
- Refractive surgery