This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Ophthalmology
Most important factor determining convergence of light rays on the retina is – 
Select and explain. 
A. Length of eyeball, B. Dioptre power of lens, C. Refractive index of cornea, D. Physical state of vitreous
C: Refractive index of cornea
Answer: C: Refractive index of cornea 
Explanation: 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 the refractive index of the two surfaces is more. In the eye, there are three such refracting interfaces:-


Air - cornea: Difference between refractive index is 0.38 (1.38 of cornea minus 1.00 of air).
Aqueous - lens.: Difference between refractive index is 0.064 (1.4 of lens minus 1.336 of aqueous).
Lens - vitreous: Difference between refractive index is 0.064 (1.4 of lens minus 1.336 of vitreous).


As air-cornea interface has a maximum difference in refractive index, maximum refraction occurs from the anterior surface of the 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 the 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 the air-tear interface.
So, remember


If the air-tear interface is there in options it is the best answer.
If it is not in options, air-cornea is the best answer.