{"id":582,"date":"2024-07-11T11:20:14","date_gmt":"2024-07-11T11:20:14","guid":{"rendered":"https:\/\/www.meniit.com\/study-material\/?p=582"},"modified":"2024-07-29T06:04:56","modified_gmt":"2024-07-29T06:04:56","slug":"defects-of-vision-and-their-correction","status":"publish","type":"post","link":"https:\/\/www.meniit.com\/study-material\/foundation\/class-x\/physics\/defects-of-vision-and-their-correction","title":{"rendered":"Defects of vision and their correction"},"content":{"rendered":"<h3 style=\"text-align: justify;\">1. Cataract:<\/h3>\n<ul style=\"text-align: justify;\">\n<li>Sometimes due to the formation of a membrane over the crystalline lens of some people in the old age, the eye lens becomes hazy or even opaque.<\/li>\n<li>This is called cataract. It results in decrease or loss in vision of the eye.<\/li>\n<li>Cataract can be corrected by surgery leading to normal vision.<\/li>\n<\/ul>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-583 size-full\" src=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/cataract.png\" alt=\"cataract\" width=\"436\" height=\"338\" srcset=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/cataract.png 436w, https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/cataract-300x233.png 300w\" sizes=\"auto, (max-width: 436px) 100vw, 436px\" \/><\/p>\n<div class=\"tableContent\">\n<h4>Table of Contents<\/h4>\n<ul>\n<li><a href=\"#Myopia-or-Near-Sightedness\"><strong>Myopia or Near-Sightedness<\/strong><\/a><\/li>\n<li><a href=\"#Hypermetropia-of-Far-Sightedness\"><strong>Hypermetropia of Far-Sightedness<\/strong><\/a><\/li>\n<li><a href=\"#Presbyopia\"><strong>Presbyopia<\/strong><\/a><\/li>\n<li><a href=\"#Astigmatism\"><strong>Astigmatism: Example and Solution<\/strong><\/a><\/li>\n<\/ul>\n<\/div>\n<p style=\"text-align: justify;\">People with normal vision can focus clearly on very distant objects. We say their far point is<br \/>\nat infinity.<\/p>\n<p style=\"text-align: justify;\">People with normal vision can also focus clearly on near objects up to a distance of 25 cm.<br \/>\nWe say their near point is at a distance of 25 cm from the eye.<\/p>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-584 size-full\" src=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/retina.png\" alt=\"retina \" width=\"637\" height=\"187\" srcset=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/retina.png 637w, https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/retina-300x88.png 300w\" sizes=\"auto, (max-width: 637px) 100vw, 637px\" \/><\/p>\n<h3 id=\"Myopia-or-Near-Sightedness\" style=\"text-align: justify;\">2. Myopia or Near-Sightedness<\/h3>\n<p style=\"text-align: justify;\">A person with myopia can see nearby object clearly but cannot see distant objects distinctly. A myopic person has the far point nearer than infinity. In a myopic eye, the image of distant object is formed in front of the retina, this defect arise due to:<\/p>\n<ul>\n<li style=\"text-align: justify;\">Excessive curvature of eye lens<\/li>\n<li style=\"text-align: justify;\">Elongation of eye ball<\/li>\n<\/ul>\n<p style=\"text-align: justify;\">This defect can be corrected by using a <strong>concave lens<\/strong> of suitable power.<\/p>\n<p style=\"text-align: justify;\">The rays of light from distant object are diverged by the concave lens so that final image is formed at the retina. If the object is very far off (i.e. u = \u221e), then focal length of the concave lens is so chosen that virtual image of the distant object is formed at the far point F of the short-sighted eye. Therefore, rays of light appear to come from the image at the far point F of the short-sighted eye and not from the more distant object.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-588 size-full aligncenter\" src=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/myopia-or-near-sightedness.png\" alt=\"Myopia or Near-Sightedness\" width=\"663\" height=\"319\" srcset=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/myopia-or-near-sightedness.png 663w, https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/myopia-or-near-sightedness-300x144.png 300w\" sizes=\"auto, (max-width: 663px) 100vw, 663px\" \/><\/p>\n<p style=\"text-align: justify;\">If x is the distance of the far point from the eye, then for the concave lens placed before the eye:<\/p>\n<p style=\"text-align: left;\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-590 size-full aligncenter\" src=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/formula.png\" alt=\"formula \" width=\"708\" height=\"77\" srcset=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/formula.png 708w, https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/formula-300x33.png 300w\" sizes=\"auto, (max-width: 708px) 100vw, 708px\" \/><\/p>\n<p style=\"text-align: left;\">Thus, myopic eye is cured against the defect by using a concave lens of focal length equal to the<br \/>\ndistance of its far point from the eye.<\/p>\n<p style=\"text-align: left;\">Focal length of the lens for a myopic person is equal to the negative value of the person&#8217;s far<br \/>\npoint.<\/p>\n<h3 id=\"Hypermetropia-of-Far-Sightedness\" style=\"text-align: justify;\">3. Hypermetropia of Far-Sightedness<\/h3>\n<p style=\"text-align: justify;\">A person with Hypermetropia can see distant objects clearly but cannot see nearby objects distinctly.<\/p>\n<p style=\"text-align: justify;\">A hypermetropic person has the near point farther away from minimum distance of distinct vision (i.e. 25 cm). In a hypermetropic eye, the image of a nearby object is formed behind the retina.<\/p>\n<p style=\"text-align: justify;\">This defects arises due to:<\/p>\n<ul>\n<li style=\"text-align: justify;\">Increase in focal length of eye lens.<\/li>\n<li style=\"text-align: justify;\">The eye ball has become too small.<\/li>\n<\/ul>\n<p style=\"text-align: justify;\">This defect can be corrected by using by <strong>convex lens<\/strong> of suitable muscles.<\/p>\n<p style=\"text-align: justify;\">The converging lens of correct focal length will cause the virtual image to be formed at the actual near point of the farsighted person&#8217;s eye.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-594 size-full aligncenter\" src=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/hypermetropia-of-far-sightedness.png\" alt=\"Hypermetropia of Far-Sightedness\" width=\"695\" height=\"148\" srcset=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/hypermetropia-of-far-sightedness.png 695w, https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/hypermetropia-of-far-sightedness-300x64.png 300w\" sizes=\"auto, (max-width: 695px) 100vw, 695px\" \/><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-medium wp-image-595 aligncenter\" src=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/correction-for-hypermetropic-eye-300x145.png\" alt=\"Correction for Hypermetropic eye\" width=\"300\" height=\"145\" srcset=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/correction-for-hypermetropic-eye-300x145.png 300w, https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/correction-for-hypermetropic-eye.png 314w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/p>\n<p style=\"text-align: justify;\">Object placed on N\u2032 which is at a distance of D. Image of N\u2032 is produced at N which is at a distance of x and which can be said as the new near point of the hypermetropic eye.<\/p>\n<p style=\"text-align: justify;\">Then, for the convex lens placed before the eye.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-597 size-full\" src=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/formulas.png\" alt=\"formulas\" width=\"701\" height=\"159\" srcset=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/formulas.png 701w, https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/formulas-300x68.png 300w\" sizes=\"auto, (max-width: 701px) 100vw, 701px\" \/><\/p>\n<p style=\"text-align: justify;\">Thus, an eye suffering from hypermetropia can be cured against the defect by using a convex lens of focal length as calculated above.<\/p>\n<h3 id=\"Presbyopia\" style=\"text-align: justify;\">4. Presbyopia<\/h3>\n<p style=\"text-align: justify;\">This defect arises with aging. A person suffering from this defect can see neither nearby objects nor distant objects clearly\/distinctly. This is because the power of accommodation of the eye decreases due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify;\">This defect can be corrected by using bi-focal lenses. Its lower part consists of a convex lens and is used for reading purposes whereas the upper part consists of a concave lens and is used for seeing distant objects.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-602 size-full aligncenter\" src=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/Presbyopia.png\" alt=\"Presbyopia\" width=\"376\" height=\"193\" srcset=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/Presbyopia.png 376w, https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/Presbyopia-300x154.png 300w\" sizes=\"auto, (max-width: 376px) 100vw, 376px\" \/><\/p>\n<h3 id=\"Astigmatism\" style=\"text-align: justify;\">5. Astigmatism:<\/h3>\n<p style=\"text-align: justify;\">A person suffering from this defect cannot focus simultaneously on both horizontal and vertical lines of a wire gauze.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-603 size-full aligncenter\" src=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/astigmatism.png\" alt=\"Astigmatism\" width=\"520\" height=\"188\" srcset=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/astigmatism.png 520w, https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/astigmatism-300x108.png 300w\" sizes=\"auto, (max-width: 520px) 100vw, 520px\" \/><\/p>\n<ul>\n<li>This defect arises due to the fact that the cornea is not perfectly spherical and has different curvatures for horizontally and vertically lying objects.<\/li>\n<li>Hence, objects in one direction are well focused whereas objects in the perpendicular direction are not well focused.<\/li>\n<li>This defect can be corrected by using cylindrical lenses.<\/li>\n<li>The cylindrical lenses are designed in such a way so as to compensate for the irregularities in the curvature of cornea.<\/li>\n<\/ul>\n<div style=\"margin: 5px; padding: 10px; background-color: #fbdfed;\">\n<h4 id=\"example-solution\" style=\"color: #cc1d74;\"><strong>Example :\u00a0<\/strong><\/h4>\n<p>A long sighted person has a minimum distance of distinct vision of 50 cm. He wants to reduce it to 25 cm. Find the focal length of lens used for correction.<\/p>\n<h4 style=\"color: #cc1d74;\"><strong>Solution :\u00a0<\/strong><\/h4>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-550 size-full\" src=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/astigmatism-2.png\" alt=\"Astigmatism\" width=\"923\" height=\"372\" \/><\/p>\n<\/div>\n<div style=\"margin: 5px; padding: 10px; background-color: #fbdfed;\">\n<h4 id=\"example-solution\" style=\"color: #cc1d74;\"><strong>Example :\u00a0<\/strong><\/h4>\n<p>Least distance of distinct vision of a long-sighted man is 40 cm. He wishes to reduce it to 25 cm by using a lens, calculate the focal length of the lens used.<\/p>\n<h4 style=\"color: #cc1d74;\"><strong>Solution :\u00a0<\/strong><\/h4>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-550 size-full\" src=\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/07\/least-distance-of-long-sighted.png\" alt=\"least distance of long sighted\" width=\"923\" height=\"372\" \/><\/p>\n<\/div>\n<div class=\"newspaper-x-tags\"><strong>TAGS: <\/strong><span><a href=\"https:\/\/www.meniit.com\/study-material\/tag\/astigmatism\" rel=\"tag\">Astigmatism<\/a><\/span><a href=\"https:\/\/www.meniit.com\/study-material\/tag\/cataract\" rel=\"tag\">Cataract<\/a><\/span><a href=\"https:\/\/www.meniit.com\/study-material\/tag\/defects-of-vision\" rel=\"tag\">Defects of vision<\/a><\/span><a href=\"https:\/\/www.meniit.com\/study-material\/tag\/hypermetropia-of-far-sightedness\" rel=\"tag\">Hypermetropia of Far-Sightedness<\/a><\/span><a href=\"https:\/\/www.meniit.com\/study-material\/tag\/myopia-or-near-sightedness\" rel=\"tag\">Myopia or Near-Sightedness<\/a><\/span><a href=\"https:\/\/www.meniit.com\/study-material\/tag\/presbyopia\" rel=\"tag\">Presbyopia<\/a> <\/div>\n","protected":false},"excerpt":{"rendered":"<p>1. Cataract: Sometimes due to the formation of a membrane over the crystalline lens of some people in the old&nbsp;&nbsp;&#8230;.<a class=\"read_more\" href=\"https:\/\/www.meniit.com\/study-material\/foundation\/class-x\/physics\/defects-of-vision-and-their-correction\" rel=\"nofollow\">Read More >><\/a><\/p>\n","protected":false},"author":5,"featured_media":1121,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"om_disable_all_campaigns":false,"rank_math_lock_modified_date":false,"footnotes":""},"categories":[219,218,217],"tags":[248,244,243,246,245,247],"class_list":["post-582","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-physics","category-class-x","category-foundation","tag-astigmatism","tag-cataract","tag-defects-of-vision","tag-hypermetropia-of-far-sightedness","tag-myopia-or-near-sightedness","tag-presbyopia"],"acf":[],"_links":{"self":[{"href":"https:\/\/www.meniit.com\/study-material\/wp-json\/wp\/v2\/posts\/582","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.meniit.com\/study-material\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.meniit.com\/study-material\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.meniit.com\/study-material\/wp-json\/wp\/v2\/users\/5"}],"replies":[{"embeddable":true,"href":"https:\/\/www.meniit.com\/study-material\/wp-json\/wp\/v2\/comments?post=582"}],"version-history":[{"count":16,"href":"https:\/\/www.meniit.com\/study-material\/wp-json\/wp\/v2\/posts\/582\/revisions"}],"predecessor-version":[{"id":1125,"href":"https:\/\/www.meniit.com\/study-material\/wp-json\/wp\/v2\/posts\/582\/revisions\/1125"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.meniit.com\/study-material\/wp-json\/wp\/v2\/media\/1121"}],"wp:attachment":[{"href":"https:\/\/www.meniit.com\/study-material\/wp-json\/wp\/v2\/media?parent=582"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.meniit.com\/study-material\/wp-json\/wp\/v2\/categories?post=582"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.meniit.com\/study-material\/wp-json\/wp\/v2\/tags?post=582"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}