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Is my line of reasoning flawed?

  1. Mar 14, 2004 #1
    Can someone tell me if I’ve gone wrong somewhere in the following line of reasoning? It’s a line of reasoning which leads me to believe that, when we look at an object underwater, it’s image is actually formed upright (instead of inverted) on our retina.

    The reason I assert that the image formed is upright is because the aqueous humour-air interface (which is responsible for the refraction which causes the image to be inverted in most cases) is replaced with an aqueous humour-water interface. Water and the aqueous humour both have a refractive index of approx. 1.34, so little or no refraction will occur when light passes from one to the other. The reason I am paying no attention to the refractive index of the cornea (approx. 1.38) is because the opposite sides of the cornea can be considered as parallel for the purposes of this experiment.
    Because the refraction occuring between the two mediums (water and the aqueous humour) in this instance is effectively negligible, rays which are parallel at the cornea can be considered to be still parallel at the lens, as demonstrated in the attached picture. Values I’ve been able to gather for the refractive power of the lens are: 20 diopters when relaxed, and 30-33 during accommodation. Therefore, parallel rays striking the lens will be focused approx. 3-5 cm behind the lens. The distance from the lens to the retina, however, is only 1.4-1.7 cm. The image of the object in the water, therefore, is upright on the retina.

    Have I gone wrong somewhere?

    P.S. To make the scenario easier to cope with, assume that the object in question is small enough for the light rays reflected off its top and bottom sides in to the eye to be parallel.
     

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    Last edited: Apr 6, 2004
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  3. Mar 14, 2004 #2

    HallsofIvy

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    Surely it has occured to you that IF your line of reasoning were correct, then objects would not focus on the retina and we could not see underwater. That's not been my experience!

    Also, I am puzzled by your statement "The reason I am paying no attention to the refractive index of the cornea (approx. 1.38) is because the opposite sides of the cornea can be considered as parallel for the purposes of this experiment." How did you decide that? It is, after all, the lens (cornea) that is responsible for normal focusing on the retina, not the aqueous humor.
     
  4. Mar 14, 2004 #3
    Yes it has occurred to me, but something else also. It seems to me that, according to the prevailing theory of visual awareness, the consequence of an image being formed upright on the retina would be that we would 'actually see' the image upside down (as our brain is supposed to re-invert images formed on the retina - right?). I've tested this a number of times by looking at objects underwater, and I don't see them upside down - I see them upright!

    Well, my notion that the opposite sides of the cornea are approximately parallel is based on photographs such as the ones located here and here. And, if they can be considered as being approx. parallel, well then I think I'm correct in saying that light travelling from water-cornea-aqueous humour will behave in the fashion depicted in the attached picture, i.e. it will be refracted through an angle x when it strikes the cornea, but will then be refracted through an angle approx. equal to x but in the opposite direction when it reaches the aqueous humour. So overall, the direction of the light ray after it leaves the cornea would be very close to the direction it was travelling before it reached the cornea.
    This is probably the point I'm least sure about; if I'm wrong, please just show me how.

    Ok, but seeing as the refractive index of the mediums on either side of the cornea are equal in this instance, and seeing as I think that is reasonable to treat the opposite sides of the cornea as being parallel, I think (again) that the behaviour of the light rays would be as depicted in the attached picture. So I don't think the refractive index of the cornea is relevant in this instance.

    I'm a non-scientist, and you're obviously far more knowledgable on these topics than myself, so I hope you don't consider my writing arrogant. If my arguments are flawed, please just point out to me where I have gone wrong. And thank you for the reply!

    Anyone else, also feel free to let me know if you think my reasoning is flawed.
     

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    Last edited: Mar 15, 2004
  5. Mar 15, 2004 #4

    russ_watters

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    The index of refraction issue is why our focus gets screwed up underwater. Other than that, our eyes work the same underwater as in air.
     
  6. Mar 15, 2004 #5
    But do you agree with my conclusion that the image of the object formed on the retina must be upright when viewing underwater (as depicted in the picture attached to my first post, i.e. underwater_vision.gif).
    I know I'm possibly beginning to sound like a broken record, but if you don't agree with me, can you just tell me why?
     
  7. Mar 15, 2004 #6

    selfAdjoint

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    Yes it has occurred to me, but something else also. It seems to me that, according to the prevailing theory of visual awareness, the consequence of an image being formed upright on the retina would be that we would 'actually see' the image upside down (as our brain is supposed to re-invert images formed on the retina - right?). I've tested this a number of times by looking at objects underwater, and I don't see them upside down - I see them upright!

    And what is wrong with this? You are underwater, and the object you are looking at is underwater, so the simple lens in your eye will put the image of the object upside-down on your retina, and your visual cortex will present it to the rest of your brain as upright. This works the same way underwater as in the air. Why wouldn't it?
     
  8. Mar 15, 2004 #7
    Here's the question you should ask yourself: does the fact of looking through a window change the way an object is percieved? Looking through water is like looking through air or looking through glass. There is no qualitative difference in the way objects are percieved if they are underwater, or on the other side of a pane of glass.
     
  9. Mar 15, 2004 #8

    krab

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    SamLuc, I think where you go wrong is in attributing the inversion to the effect of the lens. Think of your iris as a pinhole; no lens at all. The image would still be inverted. All the lens does is focus this inverted image. As you point out, the indices of refraction are so little different in the underwater case, very little focusing happens at all. That's why things are so blurry. I've looked at your illustration. Note that the object you've drawn is about the same size as your iris. Realize that objects this small are simply not visible at all under water.
     
  10. Mar 15, 2004 #9
    selfAdjoint wrote:
    My first two posts are solely dedicated to demonstrating why the image of the object on our retina is not upside-down in this instance.

    selfAdjoint wrote:
    Again, for the reason outlined in my first two posts.

    Zero wrote:
    Yes, I have verified this to myself by looking at objects underwater. We see things upright and approx. the same size in both cases. But this presents what seems to me to be a paradox: if the images formed on our retina of objects we view underwater are upright (which, according to my calculations, they are), then, according to the widely-accepted theory that our brain automatically inverts/re-inverts the image before 'presenting it to
    the rest of our brain' as selfAdjoint put it, we should actually see things upside-down. But we don't!

    krab wrote:
    Which lens? The corneal lens or the biconvex lens?

    krab wrote:
    Ok. I'm a bit confused. According to my textbooks, there are two main factors which contribute to the refractive power of the eye: (i) the cornea/aqueous humor, and (ii) the (biconvex) lens. They also state that the majority of the eye's refractive power is due
    to the cornea/aqueous humor, and that the (biconvex) lens accounts for only a small portion of the eye's refractive power. In my first two posts I have demonstrated why I think that, when viewing an object underwater, the refractive power of the cornea/aqueous humor is effectively obliterated. I also pointed out that (according to my calculations) the refractive power of the biconvex lens by itself isn't enough to invert the image. It is for these reasons that I think the image formed on our retina of an object we view underwater, which is upright to begin with, will also be upright (..and it is the reasoning I used in reaching this conclusion that I think needs to be scrutinized). Are you now telling me that the iris also contributes to the refractive power of the eye? It's the first I've heard of it, and I don't think it's true to be honest.

    krab wrote:
    I have been able to identify objects which approximate to this size underwater. But even if one were to look at a larger object underwater, so that the rays from its top and bottom sides were actually beginning to converge when they reach the eye, one should at the very least see the object diminished, if not actually upside-down.
     
  11. Mar 15, 2004 #10
    This is nonsense. Go read a book on sight and take its word for it.
     
  12. Mar 15, 2004 #11
    Ok, first of all, I have "read a book on sight", but no, I will absolutely not just "take its word for it".

    What kind of scientific approach is that? I thought that the most highly regarded method is, when experimental results conflict with theory, one questions/refines the theory. You don't simply discard/pay no attention to the results.

    The reason I started this thread is so people could critique my method of interpreting the results I have found. I am more than willing to concede that my method/line of reasoning is wrong if someone can show me how. But, again, I'm certainly not going to just take anyone's "word for it" that a present theory is correct regardless of whether or not experimental evidence is in conflict with it.

    You haven't even bothered to ask what my alternative explanation is. But of course your mind is already made up, so no new results would ever change it.

    Back up your assertion, or don't bother making it please.
     
    Last edited: Mar 15, 2004
  13. Mar 15, 2004 #12
    The do the experiment. If being underwater changed the way we see something, then a simple experiment would be to take a picture and see what you get.

    Honestly, though, there is no reasoning going on here at all, in the slightest.
     
  14. Mar 15, 2004 #13
    The point is, I guess, it is your brain that flips the image when it travels from the eye through the optic nerve. If the image hit the retina rightside-up, the brain would show everything in water upside-down. Get it?
     
  15. Mar 15, 2004 #14
    Oh my God! I have done the experiment. Me looking at an object underwater and verifying that I see it upright constitutes actually carrying out the experiment. Perhaps one of the simplest experiments ever conceived, but an experiment nonetheless.

    There is absolutely no point whatsoever in taking a picture. Firstly, the apparatus inside the camera will behave differently to the apparatus inside my eye. Secondly, consciousness is a completely personal experience. You completely miss the point!

    I don't know what you're trying to do here - bolster your reputation as a forum moderator by pasting the young heretic perhaps? - but you are the one who is being unreasonable! To recap on my line of reasoning:
    (i) Water and aqueous humour have approx. the same refractive index. Therefore, when viewing an object underwater, rays which are parallel before reaching the cornea are approx. parallel when they leave the cornea. (I clearly explain my reason for believing that the cornea doesn't significantly alter the overall direction of the light in my second post - don't be afraid to look at it now, don't just dismiss my belief as unfounded!)
    (ii) Once the refractive power of the cornea/aqueous humour has been effectively obliterated, the refractive power of the lens (which is at most 20 diopters) isn't enough by itself to invert the image (see my first post for elaboration).

    As I have already said, I am willing to accept that my line of reasoning might be flawed. In fact that's the reason I presented it here, so people could analyse and criticise it - point out any possible errors. But to assert that "there is no reasoning going on here at all" is to practice bigotry, and is absolutely unfair. You have twice knocked what I've said, and haven't once even attempted to point out where I've gone wrong.
     
  16. Mar 15, 2004 #15
    Here, let me repost this part, the relevant part, ok?


    The point is, I guess, it is your brain that flips the image when it travels from the eye through the optic nerve. If the image hit the retina rightside-up, the brain would show everything in water upside-down. Get it?
     
  17. Mar 15, 2004 #16
    This has nothing to do with consciousness, and everything to do with physics and neurology. Your brain flips EVERYTHING that hits the back of your eye. If something hits your retina "right-side up", it will always appear upside-down. Period. That's the way your brain and eye are wired together, there's nothing to debate here.


    Interesting thinking, but no dice...I'm sure you bug the pure heck out of your teachers, don't you?
     
  18. Mar 15, 2004 #17
    Yes, I do get the fact that according to the now most widely-accepted theory, this is what should happen. My acknowledgement of this fact has been a subtext of what I've said almost from the beginning. Geez!

    Another subtext of what I've been saying is that if my line of reasoning is correct, there exists evidence (which almost everyone has themselves witnessed - as almost everyone has looked at objects underwater I would imagine) that contradicts this theory. Get it?
     
  19. Mar 15, 2004 #18
    No, I really don't. Since taking a picture with any camera doesn't show an inverted image, that shows pretty authoritatively that the image does exactly the same thing as it would if it were on land. I don't understand why you would think that two different things were happening, when we see the exact same thing on land or underwater. Since our optic nerve doesn't change when we go underwater(see, it is inside your head, and doesn't get wet...) the image must be hitting the inside of your eye the same underwater as it is on land.
     
  20. Mar 15, 2004 #19
    I don't have any teachers. I'm sorry if I seem like an arrogant little s**** by the way. But I've put alot of thought into this, and I'm not going to just accept people telling me that I'm wrong unless they show me how I'm wrong.

    I've got to call it a day for now, but I might post my alternative explanation tomorrow, if yourself or anyone else is interested! Heck, I might even post if no one is interested. I promise - it can explain the paradox I think my line of reasoning exposes (although it does raise other questions).

    Thanks for putting up with me! And I think I might even have enjoyed the discussion.
     
  21. Mar 15, 2004 #20
    Well, don't strain yourself TOO much...your grasp of refraction and all that seems fairly solid, but the biology contradicts you.
     
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