Most important factor determining convergence of light rays on retina is

NEET PG Ophthalmology
#1

Most important factor determining convergence of light rays on retina is –

    1. Length of eyeball
    1. Dioptre power of lens
    1. Refractive index of cornea
    1. Physical state of vitreous

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EXP:

  • The cornea is the most important refractive surface of the eye with a refractive index of 1.376. The reason for maximum refraction lies in the fact that when light travel from one refractive medium to other, refraction will be more if the difference between refractive index of the two surface is more. In eye, there are three such refracting interfaces :-
  1. Air - cornea : Difference between refractive index is 0.38 (1.38 of cornea minus 1.00 of air).
  2. Aqueous - lens.: Difference between refractive index is 0.064 (1.4 of lens minus 1.336 of aqueous).
  3. Lens - vitreous : Difference between refractive index is 0.064 (1.4 of lens minus 1.336 of vitreous).
  • As air - cornea interface has maximum difference in refractive index, maximum refraction occurs from anterior surface of cornea.
  • Therefore, out of + 60 D (total diopter power of eye), + 44 D is provided by the cornea.
  • It is important to mention one important fact here.
  • Normally cornea is covered by tear, so actual refraction occurs at air-tear interface.
  • Tear has refractive index 1.336. Difference between refractive index at air tear interface is (0.336), maximum of all interface.
  • Therefore, maximum refraction occurs at air-tear interface.
  • So, remember
  1. If air-tear interface is there in options it is the best answer.
  2. If it is not in options, air-cornea is the best answer.