Cornea responsible for most of the eye's refraction?

[SamLuc]

Numerous texts I've read on eye anatomy/physiology claim that it is the cornea that is responsible for most of the eye's refractive power. Can someone perhaps explain to me how this could be the case?
I would have thought that, considering that the tangents to the points of entry and exit of a light ray passing through the cornea are approximately parallel, the overall refractive power of the cornea would be very little. I would have thought, therefore, that it is the aqueous humour that is responsible for most of the eye's refractive power.
Am I missing something? Can anyone help me? Is it conceivable that the texts I have been reading are actually wrong?

P.S. A diagram always helps. Please look at the one http://www.samluc.com/science/vision/images/cornea.gif . It is in the manner depicted in this diagram that I believe a ray of light would behave when passing through the cornea - assuming that the mediums on either side of the cornea had the same refractive index, as when viewing an object underwater for example, as water and the aqueous humour have approx. the same refractive index.

 

[Integral]

Your diagram shows a thin layer of material on each side of that thin layer is regions of a different material. The Cornea has air on one side fluid on the other so it does not have the second interface which redirects the ray parallel to its original direction. Other interfaces inside the eye have fluid to fluid interfaces, it would seem to me that most of the refraction will occur at the interface with the greatest change in index of refraction. This would be the cornea to air surface.

Personal experience tells me that a good spherical first surface of your eye is essential to vision. My corneas suffer a condition called Keratoconus, or conical cornea. At its worst I was seeing multiple images with a single eye, as each portion of my misshaped cornea approximated a different sphere. A different image was created for each radius of curvature. That cornea has been replaced.

 

[SamLuc]

That sounds like a highly unpleasant condition to have. I hope the replacement has made things much better for you.

But, say if you replaced the air interface with water - which has almost exactly the same refractive index as aqueous humour - wouldn't the ray of light behave as in the diagram?

 

[Integral]

If you remove the second interface, yes.

 

[SamLuc]

Thanks for your time Integral.

Integral's vote almost completely consolidates my already-strong opinion, but:
If, for any reason - e.g. the microanatomy of the cornea - people might feel that a ray of light passing from water-cornea-aqueous humour wouldn't behave in the manner depicted in the above diagram, please let me know.

 

[nautica]

The cornea is 2/3 of the refractive power. The lens is 1/3. The reason is that light is going from air to the aquoeous humor, a more dense substance. There is no major change in refractive indexs after entering.

Nautica