Refractive System Of Eye

REFRACTIVE SYSTEM OF EYE


REFRACTIVE SYSTEM OF EYE

  • Main function of refractive system of eye is to focus the light rays on the retina.
  • The cornea (especially anterior surface) is the most important refractive surface of eye.

Refractive power of cornea depend on 2 factors:

  1. Refractive index of cornea (directly related)
  2. Radius of curvature (inversely related)
  • Second important refractive structure of eye is lens with refractive index 1.3886-1.406.
  • Center (nucleus) of lens has mximum refractive index (1.46) amongst all the refractive structures of eye.
  • Third factor determining the refractive power of eye is the axial length of eyeball.
  • 1mm change in axial length leads to change of 3D in refractive power.

MEDIUM

REFRACTIVE INDEX

Air 1.000
Cornea 1.376
Aqueous humour 1.336
Lens 1.386-1.406 (maximum)
Vitreous humor 1.336

 ERRORS OF REFRACTION

  • Emetropia (optically normal eye) is defined as a state of refraction, wherein the parallel rays of light coming from infinity are focused at the sensitive layer of retina with the accommodation at rest.
  • Ametropia (a condition of refractive error) is define as a state of refraction, wherein the parallel rays of light coming from infinity are not focused on retina, rather focused either in front (in myopia) or behind (in hypermetropia) the sensitive layer of retina.
  • The ametropia can be: i) Myopia  ii) Hypertropia  iii) Astigmatism
  • Anisometropia is a term used when diopteric power in both eyes are not the same.

CAUSES OF AMETROPIA

Possible causes are:

1) Axial

  • Commonest form of ametropia (both myopia & hypermetropia).
  • In hypermetropia, there is an axial shortening of eyeball (image formed behind retina).
  • In myopia, there is an axial lengthening of eyeball (image formed in front of retina).
  • 1 mm change in axial length leads to ametropia of 3D.

1. Curvature

  • Change in curvature of cornea or lens will cause ametropia.
  • In hypermetropia, the curvature of cornea or lens is lesser than normal.
  • In myopia, there is increase in curvature of cornea or lens.
  • 1 mm change in corneal curvature leads to 6-7 D ametropia.

2. Index

  • If refractive index of optical system is low, will result in hypermetropia & high refractive index will result in myopia.

3. Positional (Due to relative position of the lens)

  • A forward shift of lens causes myopia, backward shift results in hypermetropia.
  • Absence of lens (aphakia) causes hypermetropia.

 4. Excessive accommodation

  • Due to spasm of accommodation causes myopia.

Exam Important

  • Power of reduced eye is +60 D.
  • Power of normal eye is +60 D.
  • Refractive index of vitreous humor is 1.336.
  • Refractive index of cornea is 1.38.
  • Anterior surface of cornea with maximum refractive power.
  • The refractive power of the cornea is 43D. 
  • Maximum refractive index is of Lens.
  • Highest refractive index in eye is of Centre of lens.
  • 1mm change in the axial length of the eyeball produces a change in refractive index by  3D.
  • Anisometropia is an optical state with unequal refraction of the two eyes.
  • 1 mm change in axial length leads to ametropia of 3D.
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