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
1mm change in the axial length of the eyeball produces a change in refractive index by: March 2005 
Select and explain. 
A. 1 D, B. 2 D, C. 3 D, D. 4 D
C: 3 D
Answer: C: 3 D 
Explanation: Ans. C: 3D 1mm change in the axial length of the eyeball produces a change in refractive index by 3 D. Elongation produces myopia and shoening causes hypermetropia. 1 mm change in the radius of curvature of the cornea produces a 6 D refractive change. Steeper (more curved) corneas produce myopia and flatter corneas hypermetropia.