Eye defects and lenses

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Eye defects and lenses
The eye is considered normal, or emmetropic, if parallel light rays from distant objects are in sharp focus on the retina when the ciliary muscle is completely relaxed
The normal eye contracts its ciliary muscle to provide for accomodation.

When the eye cannot function properly, the light entering the eye is not brought into sharp focus on the retina.

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Eye defects and lenses

  1. 1. Eye defects and lenses By: Ellen Pintang
  2. 2.  The eye is considered normal, or emmetropic, if parallel light rays from distant objects are in sharp focus on the retina when the ciliary muscle is completely relaxed
  3. 3. Focusing an object at close range The normal eye contracts its ciliary muscle to provide for accomodation. When the eye cannot function properly, the light entering the eye is not brought into sharp focus on the retina.
  4. 4. Refractiv e errors: It is the common errors of refraction in the eyes Causes of Refractive Errors: Irregularities in the shape of cornea Abnormal size or shape of eyeball itself inability of the lens to focus
  5. 5. How can this be corrected?  These refractive errors can be fully corrected with eyeglasses or contact lenses
  6. 6. Common Eye Defects
  7. 7. Hyperopia or Farsightedness This condition is due to either the eyeball is too short or, sometimes, a lens that is too weak. How does hyperopia happen? It happens because parallel light rays are not bent sufficiently by the relaxed lens system to come focus by the time they reach the retina. The ciliary muscles of the eye are not strong enough to contract, resulting to the lens not being convex enough to allow for sharp focus.
  8. 8. Myopia or Nearsightedness Caused by having an eyeball that is too long , or having too much refractive power in the lens system of the eye. In myopia, the ciliary muscle is completely relaxed and the light rays coming from distant objects are focused in front of the retina. A myopic person has no mechanism to focus distant objects sharply on the retina.
  9. 9. Astigmatism Caused by irregularly curved cornea that distorts the focus entering the eye. This defect causes the visual image in one plane to focus at a different distance from that plane at right angles. Because of this condition, the light rays passing through an astigmatic lens do not all come to a common focal point. Thus blurred image is perceived by an astigmatic person.
  10. 10. Correcting Common Eye Defects
  11. 11. This condition can be corrected by adding refractive power with a convex lens in front of the eye. How to correct Hyperopia or Farsightedness Convex lens are converging lenses. If the light rays converge to a focal point on the retinal surface it creates a focused image.
  12. 12. In myopia, the eyes of a myopic person has an excessive refractive power that can only be neutralized by concave lenses. How to correct Myopia or Nearsightedness Placing a diverging lens in front of the eye diverges the light rays just enough such that the rays converge on the retinal surface where a focused image is created.
  13. 13.  Find a spherical lens by trial and error that corrects the focus in one of the two planes of the astigmatic lens of the eye.  After the correct spherical lens has been determined, an additional cylindrical lens is used to correct in the remaining plane. How to correct Astigmatism
  14. 14. Contact Lenses: Are transparent materials that are fitted against the interior surface of the cornea. This lenses are held in place by a thin layer of liquid (tears) that fills the space between the contact lens and anterior surface of the eye.
  15. 15. How does it works?  The refraction of the cornea-lens system of the eye is neutralized.  The refraction is done by the anterior surface of the contact lens.
  16. 16. Advantages  It turns with the eye and thus gives a broader field of vision  Has very little effect on the size of the object the user sees. Because the glasses are placed several centimeters in front of the eye, it affects the size of he image, resulting in slightly bigger or smaller images

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