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Thread: see-through finger?

  1. #1 see-through finger? 
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    EDIT:
    It seems my explanation wasn't really clear, so I've replaced it with a better one (I hope):
    if you put your finger close to one eye while closing the other one, and make it cover up a word on your computer screen when focusing on the finger, you will then be able to see the word again if you focus on your screen. This means that the translucent edge of your finger when unfocused is going inwards, since it allows light trough that was previously blocked by your focused finger. (DO try the experiment, as it's hard to understand just by reading it)

    How is this explained?

    Thanks!


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  3. #2  
    Veracity Vigilante inow's Avatar
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    I think you might be describing a similar effect to parallax. My understanding is that this is perceptual... There is different information being combined in your brain by your eyes. One eye sees it one way, and the other eye sees it another, and the effect you are describing is merely the outcome of your brain trying to interpret the different visual signals.


    Here's a decent article on it:
    http://en.wikipedia.org/wiki/Parallax


    That one relates more to astronomy, but it is to the visual perception part to which I'm referring.
    http://faculty.washington.edu/chudler/pertr.html


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  4. #3  
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    I assume you are only looking with one eye, or else you have a very strong eye dominance so that you only see the image with one eye and the other is faded out. Otherwise, you would see not two fingers but four.

    So, looking at it with one eye, your pupil diameter has a certain width, so that even when the image crosses over the center of your vision, the light coming in past it through the edge of the pupil is still visible.
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  5. #4  
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    @ inow: thanks, but the effect happens when i'm looking with one eye, so it's not parallax

    @Harold: i'm indeed looking with one eye, so I think what you explained about the pupil size is the answer, but I need a bit more information to fully get it..
    Could you elaborate a bit?

    thank you!
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  6. #5  
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    Yes, I didn't explain that very well.

    Take a look at the figure here.
    http://en.wikipedia.org/wiki/Lens_(optics)
    part way down the page where it explains the thin lens formula 1/S1 +1/S2 = 1/f
    Let's say the object is the far away finger and the real image is at your retina. Now imagine that another object, the close finger, is introduced from below. It will be out of focus. It will first obscure the ray that comes from the object through the bottom of the lens to the retina. Then as you move it higher, it will obscure more and more of the rays. This will be seen as the blurry edge of the close finger gradually dimming then finally blocking out the object. By the time it is to the point of completely blocking it out, it is in a line from the top of the object to the top edge of the lens aperture.

    Now you change the focal length of the lens by focusing on the near object. The near object is in focus, so the fuzzy edge is no longer there, and the far object is obscured.
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    i'm not sure that is what i'm looking for, but i appreciate you trying to help me out!

    i feel we are close to the actual answer, i'll try to pinpoint my problem a bit more:
    maybe i just dont get blurryness
    I understand that, because your lens is not properly adjusted, the light emitted from one single point arrives on your retina on multiple points. This accounts for the blur. So far so good. But it seems that an edge that is blurry, also becomes translucent, thereby reducing the surface the unblurred object would take up on your retina...

    It's not that easy to explain well, but i hope you see what i mean now!
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  8. #7  
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    First, thank you, Harold, for helping to correct my mistaken guidance. I'm glad you saw my error and jumped in to help correct it.


    To pitou777 - When something is blurry, there is generally more scattering of the light. It's more diffuse, and the light tends to be less concentrated... less dense... more dispersed. For that reason, doesn't it make sense that it's somewhat opaque or translucent? I mean... the photons are more scattered, meaning there is more free space between them, meaning that photons from objects beyond the point of focus are more likely to get through.

    It's like the difference between looking through a black garbage bag versus a sheet of thin linen.
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    It makes sense, as long as the material you're looking through is always translucent..
    But the thing is, my finger isn't. Yet if you put your finger in front of one eye, and make it cover up a word on your screen, you will still be able to see the word if you focus on your screen. So even though the edge of your finger acts as a translucent material, it still isn't.

    EDIT: I tought about it some more, and maybe this is the answer: the covered up word still reaches your lens, but is refracted in your eye above your retina, so you can't actually see the word. Then when focusing on the screen, your lens changes shape, and now the word does reach your retina, so you can now see the word.

    Does that make sense?
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  10. #9  
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    Quote Originally Posted by inow
    First, thank you, Harold, for helping to correct my mistaken guidance. I'm glad you saw my error and jumped in to help correct it.
    I didn't actually see your error, I was composing my message while you were posting. The binocular vision explanation was the first thing that popped into my mind as well. It just didn't quite seem to explain what pitou777 was describing.
    Quote Originally Posted by pitou777
    I tought about it some more, and maybe this is the answer: the covered up word still reaches your lens, but is refracted in your eye above your retina, so you can't actually see the word. Then when focusing on the screen, your lens changes shape, and now the word does reach your retina, so you can now see the word.
    I think you are on the right track by thinking about the lens changing shape. When you focus on the object beyond, the image formed by the closer object is behind the retina. The blurry area is the area on the retina where the rays from the edge of the image strike, but do not quite converge. When you focus on the close object, your crystalline lens becomes more convex, shortening the focal length. I'll be back later today or tomorrow with a more detailed explanation of the effect you have been talking about. No time to work it out right now.
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  11. #10  
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    The edges of the close object (finger) are blurred due to the eye being out of focus, but the light from the background (distant object) is in focus. So the light from the edges of the close finger is being scattered amongst the in-focus background image, giving the appearance of it being translucent. The scattered light of the out of focus foreground object is interfering with the light from the background image that is being focused on, giving the impression of translucent edges. No?
    Its probably a combination of optic and cognitive effects.

    I just tried it, and I get what you mean, it is a bit odd. Got a couple of odd looks from workmates as well though. haha.
    'Aint no thing like a chicken wing'
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  12. #11  
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    thanks for the help so far :wink:

    I understand why the edge is translucent (or seems to be), but not why the transparent part goes inwards, thus making previously covered things visible.

    @harvestein: glad to hear you get what i mean, hehe

    @Harold and Inow: i think you are explaining another part of the problem,
    did you try my last explanation (with the covered word on your screen)? You really have to try it to understand where my problem is!
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  13. #12  
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    but not why the transparent part goes inwards, thus making previously covered things visible.
    I hadnt noticed that until now. I cant really suss that out either actually. better start reading up on optics.

    EDIT: i just got an instant message from a colleague across the room saying "whats with the finger"

    think ill leave the experiments fow now. :-D
    'Aint no thing like a chicken wing'
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  14. #13  
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    Quote Originally Posted by harvestein
    EDIT: i just got an instant message from a colleague across the room saying "whats with the finger"

    think ill leave the experiments fow now. :-D
    haha, that is awesome! you better tell them what it's all about, maybe they can help out :P

    EDIT: by the way, the inwards translucent-thingie is basically what my question was all along (because that's why you can suddenly see more on your screen), but i guess my explanations weren't that good
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    ok, I think i now get it, or almost. (red part i'm not sure)
    I found it easier to understand with following example:
    If you're looking at, say, a distant church tower, through a wire fence, the wire will obscure a part of the tower. But when focusing on the tower, parts that are behind the wire still reach your lens, but they reach it on the sides. your lens bends this light so that you can actually see the whole tower, even if there's wire in front of it. this is also why the translucent edge is going inwards.
    Now, when you're NOT focusing on the tower (and i'll need confirmation on this, i'm not sure), the parts behind the wire STILL reach your eye, but it's too blurry and I think the brain filters that out, to get a clear image of the wire.

    Did I get it?

    P.S.: Harold, I realize you were probably right from your first post on, but I guess i didn't get it
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    so? Am i correct?
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    Yes, I think you've got it.
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  18. #17  
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    Quote Originally Posted by pitou777
    so? Am i correct?
    I like your similar problem with the wire, or chain-link fence. If you focus on something distant, the wire is a blur and you can see what's behind it. Focusing on the wire obscures a bit of it. That's something I can relate to. I'll have to experiment.

    Have you tried ray tracing in geometric optics to see if it works?
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  19. #18  
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    i haven't tried that, but i've done some tought experiments, i think it's correct ^^
    it makes sense that, as long as your pupil is wider than the wire, you can catch some light from behind the wire.

    But now, a new question rises: Does this mean that in the dark, you'll be able to see behind larger obstacles? And in bright light, the wire will obstruct your view?
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  20. #19  
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    Quote Originally Posted by pitou777
    i haven't tried that, but i've done some tought experiments, i think it's correct ^^
    it makes sense that, as long as your pupil is wider than the wire, you can catch some light from behind the wire.

    But now, a new question rises: Does this mean that in the dark, you'll be able to see behind larger obstacles? And in bright light, the wire will obstruct your view?
    Definitely, if you try to look through a screen in a lighted room, it is much harder than it is in a dark room.
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  21. #20 Re: see-through finger? 
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    Quote Originally Posted by pitou777
    if you put your finger close to one eye while closing the other one, and make it cover up a word on your computer screen when focusing on the finger, you will then be able to see the word again if you focus on your screen.
    I picked out a word on the screen.

    I closed 1 eye.

    I then focused on my finger allowing my finger to cover the word I picked out , while holding it apx 2 inches away from my eye.

    when I looked up to focus on the screen I noticed my finger appeared to be moving down
    and I could see the word.

    I think its a matter of line of sight.

    I might be wrong.

    heres a good optical illusion

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  22. #21  
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    @pitou777:
    Could you specify what you are doing, please?
    Since it somehow doesn't work out when I try it.
    I focus on my finger while covering up a word, then focus on the screen and still can't see the word... :-D
    I am.
    You can't deny it.
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  23. #22  
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    Quote Originally Posted by mastermind
    @pitou777:
    Could you specify what you are doing, please?
    Since it somehow doesn't work out when I try it.
    I focus on my finger while covering up a word, then focus on the screen and still can't see the word... :-D
    Mastermind, try this. Take a thin object, like a straightened out paper clip, or the tip of a pencil. Something thinner than the width of the pupil of your eye. While focusing on the computer screen, try to block out a letter on the screen. It can't be done, because the light still reaches your eye by going past the paper clip, through the edge of your lens, and then is refracted to the center of your retina. If you focus on the paper clip, it will block out the image on the screen. The computer screen image will still reach your retina, but will be out of focus, so the in focus image will appear to block it out.

    An object that is thicker than your pupil can be used to totally blot out a letter on the screen, no matter what you focus on. That's because there is no longer any path from the letter on the screen to your retina.
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