Tham khảo tài liệu 'saladin anatomy and physiology the unity of form and function episode 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 16. Sense Organs I Text Saladin Anatomy Physiology The Unity of Form and Function Third Edition The McGraw-Hill Companies 2003 Chapter 16 Sense Organs 619 Table Common Defects of Image Formation a b c Figure Two Common Visual Defects and the Effects of Corrective Lenses. a The normal emmetropic eye with light rays converging on the retina. fa Hyperopia far-sightedness and the corrective effect of a convex lens. c Myopia near-sightedness and the corrective effect of a concave lens. Presbyopia Reduced ability to accommodate for near vision with age because of declining flexibility of the lens. Results In difficulty In reading and doing close handwork. Corrected with bifocal lenses. Hyperopia Farsightedness a condition in which the eyeball is too short. The retina lies in front of the focal point of the lens and the light rays have not yet come into focus when they reach the retina see top of fig. b . Causes the greatest difficulty when viewing nearby objects. Corrected with convex lenses which cause light rays to converge slightly before entering the eye. Myopia Nearsightedness a condition in which the eyeball is too long. Light rays come into focus before they reach the retina and begin to diverge again by the time they fall on it see top of fig. c . Corrected with concave lenses which cause light rays to diverge slightly before entering the eye. Astigmatism Inability to simultaneously focus light rays that enter the eye on different planes. Focusing on vertical lines such as the edge of a door may cause horizontal lines such as a tabletop to go out of focus. Caused by a deviation in the shape of the cornea so that it is shaped like the back of a spoon rather than like part of a sphere. Corrected with cylindrical lenses which refract light more in one plane than another. Chapter 16 rather its purpose is to absorb light that is not absorbed first by the receptor cells and to prevent it from degrading the visual image by reflecting back into the
