View Full Version : Cornea responsible for most of the eye's refractive power?
Hi. This is my first post here. Site looks very cool; can see myself spending some time here, if I have it to spare. Anyway, to the point of the thread: I'm a bit puzzled over something.
Numerous texts I've read on eye anatomy/physiology claim that is the cornea that is responsible for most of the eye's refractive power. Can someone please 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 light rays passing through the cornea are appxoximately parallel, the overall refractive power of the cornea would be very little. I would have thought, therefore, that light passing through the cornea would behave in the manner depicted in THIS (http://www.samluc.com/science/vision/images/cornea.gif) diagram (..assume that the mediums on either side of the cornea have the same refractive index, as would be the case when underwater for example, as water and the aqueous humour have approx. the same refractive index.)
It seems to me that it is the aqueous humour, as opposed to the cornea, that is responsible for most of the refractive power of the eye. So I don't understand why most of the texts I'm reading state otherwise.
Can anyone help me out? Am I missing something? Is it conceivable that the texts I've read are wrong on this point?
P.S. I think my treating of the opposite sides of the cornea as approx. parallel is quite reasonable, and is based on pictures you can see for yourself here (http://www.iovs.org/cgi/content/full/40/10/2442/F1),here (http://www.siumed.edu/~dking2/intro/IN022B.htm) and here (http://www.buzard.com/eyediseases/dystrophies/fuchs/fuchs.htm).
Hi SamLuc, and welcome to the forums!
I would have guessed that the refractive index of the cornea is much the same as that of the aqueous humour. So there would be significant refraction at the air-cornea boundary, but little or none at the cornea-humour boundary.
Oh... and with a water-cornea interface, images on the retina will be upside down but unfocused. And still the same size as with an air-cornea interface.
Here's a diagram that shows (very roughly) how light is refracted passing through the eye in the case of air viewing and water viewing. It shows the effect on the retinal image when there is little or no refraction at the cornea surface.
The circles in the retinal image in the second case (underwater viewing) indicate the extent that the red and blue dots are unfocused. Notice that the orientation of the image is unchanged, as is the distance between the circle centres.
The eye-object distance is unrealistically small in this diagram, but it helps show what's going on.
04-06-04, 12:36 AM
This thread reminded me of a program I watched on one of the science channels a few
months ago. It was about the sea-gypsy childrens' ability to focus their eyes underwater. Charts were set up in front of an underwater camera and the sea-gypsy
children showed an amazing ability to discern lines and shapes on the charts. Their pupils would constrict to tiny points as viewed by the cameras. From a study:
"Intrigued by tales from a colleague who had observed the food-collecting prowess of sea-gypsy children, vision researcher Anna GislÚn of Lund University in Sweden decided to investigate how such kids can pick out small objects while diving without goggles. Since many sea-gypsy tribes live on boats in remote areas and dislike strangers, GislÚn and her colleagues had to find a tribe willing to be studied. The researchers eventually worked with the Moken, a tribe living in the archipelago along the west coasts of Burma and Thailand."
Thanks for the welcome. Well, there's obviously no flies on you people - you've managed to guess what I was thinking before I told you myself. The suspicion that the image formed on the retina when viewing underwater might not be inverted/the same size was actually what led me to study image formation in the eye further. Anyway, you're telling me that the image is not inverted, and is the same size. Fair enough, I'll have to go away and think about your explanation of what's going on, but, in the mean time, here's my take on what's going on.
From discussing this with other people, I've gathered that the difficulty people have in accepting that the image formed on the retina is actually significantly different (if it is different, and it seems quite likely to me that it is - but I'm not disregarding your explanation Pete) is due to the fact that: when they look at objects underwater, they see them the same way - same size & same orientation - albeit unfocused. One can readily verify this to themselves by simply submerging their eyes in a basin of water and looking at any sort of object inside the basin.
I'm going to stick my neck out here, and say something terribly cheeky - quite CRAZY infact. This line of reasoning - if the image formed on the retina is upright/a different size, we should actually see it inverted/a different size, as our brain automatically inverts all images formed on the retina - lies on the assumption that the 'act of seeing' actually takes place inside the brain. I propose that it can actually take place outside the brain; at some stage before light from an image reaches the retina to be more specific. I also propose that visual perception occurs when the superposition of states of the quantum entities being observed - photons - collapses; the cause of this collapse being (after the initial measurement by the molecules of retinal in the photoreceptors) the registration of the visual information in V1 (striate cortex) and beyond - V2, V3 etc.
This latter, rather wild, statement deserves further elaboration. Ok. There are two relevant points that I would like to make. The first is that, the photons in our environment, until we open our eyes and look at them (or until some other process that qualifies as a measurement is carried out on them), exist in a superposition of states. I don't see how one can avoid this conclusion. The second is that, in my opinion, a measurement is exactly what occurs when a photon is absorbed by a molecule of retinal in the photoreceptors. My reason for saying this is that absorption of the photon causes a conformational change in retinal - if this doesn't qualify as a measurement I don't know what does. And if you feel that this isn't enough for collapse of the superposition of states of the photon, then I hope you will concur with my opinion that registration of the photon's existence in the mind of a conscious observer does, and this is exactly what happens when the visual information collected at the retina is registered in the visual cortices.
Now it is my turn to anticipate people's thoughts. 'But what about dreaming?' you might ask. The images/movies we see while dreaming certainly would seem to contradict my hypothesis on first analysis - after all, dreaming surely takes place inside the brain - but I personally, after giving it further analysis, don't think that this is necessarily the case. Let me explain. I have just proposed that the existence of 'photons' are a prerequisite for visual perception to take place, yet I know of no mechanism inside the brain which can create photons, at least not in the extremely organised fashion which would be necessary for what we see while dreaming. So I make a further proposal, which is this: just as the retinal molecules in photoreceptors at the back of our eye have the ability to absorb photons, they also have the ability to emit/create photons. In fact, it seems to me that: just as isomerization of retinal from 11-cis-retinal to all-trans-retinal corresponds to the absorption of a photon, changing back from all-trans-retinol to 11-cis-retinol - which is a necessary step for rhodopsin regeneration - surely corresponds to the emission of a photon. Big deal you might say; this proves nothing; because, although information can travel from retina to cortex, information can't travel the other direction; therefore no instructions can be transferred to the retina to cause the 'highly-organised photon-emission' needed for dreaming. Again, I suggest that this is not necessarily true. Information can and does reach the retina from inside the brain. What are referred to as 'centrifugal fibers' have been traced as far as the inner nuclear layer (layer directly beside photoreceptor cells, in which molecules of retinal are contained) in human retinas, where they disappear (note: 'disappear', not necessarily 'end'). Read THIS (http://webvision.med.utah.edu/fbloops.html#centrifugal) for further information.
I would also propose that these principles be applied to the auditory system. It is well-known at this stage that efferent fibers synapse on the outer hair cells in the cochlear basilar membrane, and that the micromechanical properties of the basilar membrane can therefore be controlled from 'inside the brain'. I'm going to stick my neck out again, and suggest that this is what is responsible for what we 'hear' when we are dreaming. I've never actually got the chance to test these hypotheses - although I can think of some easily-performable experiments - but if anyone can refer me to experiments/research which contradicts them, please do so; it might even curb my heretical yodels, for the time being. ;)
P.S. I'd seriously like to hear from people who are willing to engage in a rational, logical discussion of this issue - i.e. people who won't simply try to ram their opinions down my throat. I've discussed it in other forums, but only certain aspects of it, e.g. dreaming was never mentioned etc.
And Pete, I will try to get back to you on what you've said.
P.P.S. 2inquisitive, it seems to me that, if the children of these tribes really do have superior visaul acuity underwater - which seems to be the case - then the refractive power of their lenses must be stronger than those of most people. The maximum refractive power of (normal people's) lenses during accomodation is 30-33 diopters, which isn't enough to focus objects by itself when viewing underwater. So I find that information quite interesting. I've wondered how the visual system of fish works also. As of yet haven't got a chance to study it though.
Thanks for your time guys!
04-06-04, 09:22 AM
Just a quick comment: Images formed by the eye underwater are inverted on the retina with respect to their objects, just as they are in air. If they were not, we would, as you say, perceive objects under water to be upside-down.
Anyway, you're telling me that the image is not inverted, and is the same size
Please look at the diagram again. My explanation my have been ambiguous - I apologize. By "unchanged" I meant "unchanged relative to the eye-air image", as should be clear from the diagram.
The orientation and size of the image on the retina is the same when the eye is in water as it is when in air. In both cases, the image on the retina is inverted relative to the original object
What I'm about to say might be preemptive... It might also appear insulting. If you do find it offensive, please let me know and I will remove the following paragraphs.
I know you've spent a lot of time thinking about this, a I do sympathize with how difficult it is to let a long-cherished idea go. However, I really hope that you are strong enough to step back from your idea and look it it objectively in light of what you learn.
If your idea is actually flawed (this is a rel possibility for any idea!) and you continue to cling to it anyway, you're throwing good investment after bad, and heading down the road to becoming an embittered crackpot.
Having said that, I'm happy to spend some time discussing the diagram I drew above. If you think it provides an incorrect picture of what happens, or if you think my interpretation of it is wrong, then tell me! Maybe I am wrong. Hopefully we can talk it out and come to a consensus.
it seems to me that, if the children of these tribes really do have superior visaul acuity underwater - which seems to be the case - then the refractive power of their lenses must be stronger than those of most people.
If their pupils contract far enough, the focussing power of the eye is irrelevant - just like a pinhole camera works without any lens.
The difference appears to be that the low-light dilation reflex is somehow overridden, which is pretty astounding to me.
Pete, I'm not insulted at all. And I'm actually very glad you pointed this out to me. I've been thinking alot about your diagram, and think I have managed to get my head around it. And yes, it makes absolute sense to me. In fact, that's the very reason I logged back on - to acknowledge the error in my reasoning. So, to yourself and James R: yes, I think you are absolutely right. I actually feel like a bit of an ass now...but anyway.
However, having said that, I'd like to point out that the hypotheses I proposed in my last post don't actually rely on the afore-mentioned (incorrect) reasoning - i.e. that the refractive power of the front of the eye is effectively obliterated when viewing an object underwater, and that the image formed on the retina is therefore of a significantly different orientation. That reasoning was instrumental in my conceiving of these hypotheses, but it isn't actually necessary. So, if yourself or anyone else has the time, and feels like it, feel free to offer a critique of these hypotheses. I've put them in bold font.
You're probably right about the reasons for the superior visual acuity in the children of that fishing tribe as well. I have to confess that I didn't actually think about it that much.
Anyway, thanks again for your time.
04-06-04, 11:39 PM
The fact that we can see under water at all tell us that some refraction must still be occuring. I suspect that the fluid in the eye has a slightly different refractive index to that of plain water, and (probably more importantly in this case) so does the eye's lens.
I think that most physicists would find your quantum wavefunction collapse theory fairly non-controversial. I agree with you that a photon interacting with the retina of the eye would constitute a quantum "measurement" of the photon, which would collapse the wavefunction.
I'm pleased that you find these notions (or at least some of them) plausible James R. That goes a considerable way towards re-bolstering my confidence.