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  3. 3. <ul><li>Its lens forms an image on a light-sensitive screen called the retina. </li></ul><ul><li>Light enters the eye through a thin membrane called the cornea. </li></ul><ul><li>Cornea forms the transparent bulge on the front surface of the eyeball. </li></ul><ul><li>The eyeball is approximately spherical in shape with a diameter of about 2.3 cm. </li></ul>PARTS AND FUNCTIONS OF A HUMAN EYE <ul><li>The human eye is like a camera. </li></ul>
  4. 4. <ul><li>Most of the refraction of light rays entering the eye occurs at the outer surface of the cornea. </li></ul><ul><li>The crystalline lens merely provides the finer adjustment of focal length required to focus objects at different distances on the retina. </li></ul><ul><li>Iris is a dark muscular diaphragm behind the cornea and it controls the size of the pupil. </li></ul><ul><li>The pupil regulates and controls the amount of light entering the eye. </li></ul><ul><li>The eye lens forms an inverted real image of the object on the retina. </li></ul>
  5. 5. <ul><li>The eye lens is composed of a fibrous, jelly-like material. </li></ul><ul><li>Its curvature can be modified to some extent by the cilLiary muscles. </li></ul><ul><li>The change in curvature can thus change the focal length of the lens. </li></ul><ul><li>The retina is a delicate membrane having enormous number of light-sensitive cells. </li></ul><ul><li>The light-sensitive cells get activated upon illumination and generate electric signals. </li></ul><ul><li>These signals are sent to brain via optic nerves. </li></ul>
  6. 6. POWER OF ACCOMMODATION The ability of the eye lens to adjust its focal length is called accommodation. The eye lens is composed of a fibrous, jelly-like material and its curvature can be modified by the ciliary muscles. Hence, the focal length can be changed as per the requirement. When the muscles are relaxed, the lens becomes thin. The radius of curvature and hence the focal length increases. This enables us to see the distant objects clearly. When we look at the objects closer to they, ciliary muscles contract decreasing the radius of curvature and hence the focal length. This enables us to see the nearby objects clearly.
  7. 7. LEAST DISTANCE OF DISTINCT VISION (LDDV): The minimum distance, at which objects can be seen most distinctly without strain, is called Least Distance of Distinct Vision(LDDV). For a normal eye, LDDV is 25 cm.
  8. 8. BENEFITS OF TWO EYES FOR VISION There are several advantages of our having two eyes instead of one. It gives a wider field of view . a human being has a horizontal field of view of about 150 degree with one eye and of about 180 degree with two eyes . The ability to detect faint objects is , of course , enhanced with two detectors instead of one . Our two eyes our positioned on the front of our heads , and it thus reduces our field of view in favour of what is called stereopsis . Our brain combines the two images into one.
  9. 10. MYOPIA OR SHORT-SIGHTEDNESS OR NEAR-SIGHTEDNESS <ul><li>A person with myopic eye can see nearby objects clearly but cannot see distant objects distinctly. </li></ul><ul><li>Such a person may clearly see upto a distance of a few metres. </li></ul><ul><li>In myopic eye, the image of a distant object is formed in front of the retina and not on the retinal itself. </li></ul><ul><li>This defect may arise due to : </li></ul><ul><li>excessive curvature of the eye lens (short focal length of the eye lens) </li></ul><ul><li>or </li></ul><ul><li>(ii) Elongation of the eyeball. </li></ul><ul><li>Myopia can be corrected by using a concave lens of suitable power(focal length). </li></ul>
  10. 11. Myopic Eye Near Point MYOPIC EYE CORRECTED WITH CONCAVE LENS Normal Eye LDDV = 25 cm O LDDV = 25 cm I O LDDV = 25 cm O I I O LDDV = 25 cm I O LDDV = 25 cm I I
  11. 12. HYPERMETROPIA OR LONG-SIGHTEDNESS OR FAR-SIGHTEDNESS A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. Such a person may has to keep a reading material much beyond 25 cm from the eye for comfortable reading. In hypermetropic eye, the image of a nearby object is formed behind the retina and not on the retinal itself. This defect may arise due to : (i) long focal length of the eye lens or (ii) Very small size of the eyeball. Hypermetropia can be corrected by using a convex lens of suitable power (focal length).
  12. 13. Hypermetropic Eye Near Point HYPERMETROPIC EYE CORRECTED WITH CONVEX LENS Normal Eye LDDV = 25 cm O LDDV = 25 cm I O O LDDV = 25 cm I I LDDV = 25 cm I O LDDV = 25 cm O I I
  13. 14. PRESBYOPIA <ul><li>The power of accommodation of the eye usually decreases with ageing. </li></ul><ul><li>People can not see nearby objects comfortably and distinctly without corrective eye-glasses. </li></ul><ul><li>This defect is called presbyopia. </li></ul><ul><li>It arises due to : </li></ul><ul><li>gradual weakening of the ciliary muscles and </li></ul><ul><li>diminishing flexibility of the eye lens. </li></ul><ul><li>Sometimes, a person may suffer from both myopia and hypermetropia. Such </li></ul><ul><li>people require bi-focal lenses which consists of both concave and convex </li></ul><ul><li>lenses. The upper portion is concave for distant vision and the lower portion </li></ul><ul><li>is convex for near vision. </li></ul>