How eyes see.... Process of vision begins when light rays that reflect off objects and travel through the eyes optical system are refracted and focused into a point of sharp focus. For good vision, this focus point must be on the retina. The retina is the tissue that lines the inside of the back of the eye, where light-sensitive cells (photoreceptors) capture images in much the same way that film in a camera does when exposed to light. These images then are transmitted through the eyes optic nerve to the brain for interpretation. Just as a cameras aperture (called the diaphragm) is used to adjust the amount of light needed to expose film in just the right way, the eyes pupil widens or constricts to control the amount of light that reaches the retina. In dark conditions, the pupil widens. In bright conditions, the pupil constricts. Causes of Refractive Errors The eyes ability to refract or focus light sharply on the retina primarily is based on three eye anatomy features: 1) the overall length of the eye, 2) the curvature of the cornea and 3) the curvature of the lens inside the eye. Eye Length: If the eye is too long, light is focused before it reaches the retina, causing nearsightedness. If the eye is too short, light is not focused by the time it reaches the retina. This causes farsightedness or hyperopia. Curvature of the Cornea: If the cornea is not perfectly spherical, then the image is refracted or focused irregularly to create a condition called astigmatism. A person can be nearsighted or farsighted with or without astigmatism. Curvature of the Lens: If the lens is too steeply curved in relation to the length of the eye and the curvature of the cornea, this causes nearsightedness. If the lens is too flat, the result is farsightedness. More obscure vision errors, known as higher-order aberrations, also are related to flaws in the way light rays are refracted as they travel through the eyes optical system.
Posted on: Wed, 24 Sep 2014 03:14:39 +0000
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