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What is Shortsightedness (myopia)?

What is Shortsightedness

Myopia consists of a malrelation between the length of the eye and the refractive power of the cornea (dioptres). The cornea is the transparent layer of the eye on top of the iris (black, curved line). In most cases, the eye is too long and the refractive power of the cornea too strong, so that the rays of light which reflect a distant object are bundled in a point in front of the retina (see focal point of black rays of light in pictures on the right). This produces a blurry image on the retina and results in inaccurate vision at distance. The retina - not to be confused with the cornea - is the light-sensitive layer at the back of the eye, which takes up the optical depiction. The retina is not explicitly illustrated in the pictures, and needs to be imagined in the point where the rays of light are supposed to combine to a focal point (focal point of red rays of light). The refractive power of the eye for light depends on the curvature of the corneal surface. Specifically, the dioptres (refractive power) are determined by the radius of the corneal curvature and are reversely proportional to the radius of the cornea. A short corneal radius corresponds to a high refractive power, which means that the eye is relatively myopic. The traditional method to correct myopic vision is to place appropriate glasses in front of the eye (red rays of light, top) or contact lenses onto the eye (red rays of light, middle). This helps to correct a person’s vision by shifting the focal point backwards to the retina (blue arrow). The rays of light are already diverted before they penetrate the eye and enable the eye to produce an accurate image of a distant object. Another option is to modify the too long corneal radius by changing the curvature of the corneal surface accordingly (refractive surgery). This is illustrated in the bottom picture: the initially stronger curvature of the cornea (black) is flattened (red, curvature radius of the cornea is increased). In a myopic eye, the centre of the cornea needs to be flattened to neutralise the excessive refraction power of the eye for light and shift the focal point backwards towards the retina (blue arrow).

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