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Light seems to bend around objects but why?

  1. May 25, 2010 #1
    Hello everyone,
    First of all I am new here and I hope I came to the right place :)

    I have been troubled by two observations I have about light, I come into difficulty to find the right words but I will try:

    First of all here is a picture that may help:

    Observation 1:
    With one of my eyes open I look at a pencil eraser (it works with other objects too) at a distance of 25cm, at the background there is a couch at a distance of about 4-5m.

    The observation is that when I move another object (lets say a pen cap) very close to my eye then the eraser edge seems to be pulled outwards toward the pen cap while the background couch stays relatively* intact. (view A & B)

    Then there is observation 2:
    I align my eye sight as much parallel as possible to the tables surface ( the table that the eraser is on ) also near the right edge of the table so that I can use the further corner point of the table as a precise aiming help, to aim exactly at the couches upper edge.

    By moving the pen cap in front of my eye towards the couch edge the couch edge seems to be "pressed" downwards ( away from the pen cap ). (view C & D)

    I understand that it may be difficult for my post to make sense but I hope that it does as I have great difficulty to explain it in written form in a non mother tongue and a science I have not any worth mentioning knowledge.

    I hope someone can understand and help me understand what and why is it happening.

    Thanks from now for any help :)
  2. jcsd
  3. May 25, 2010 #2
    Welcome to Physics Forums!

    Are your eyes focusing on the pen cap when you do this?
  4. May 25, 2010 #3
    Thanks for welcoming me :)
    At observation 1 it only works when I focus on the couch ( unlimited ), it doesn't work when focusing on the cap or the eraser.
    At observation 2 it seems to work when I am looking at the couch or the table further edge but it doesn't work when I focus on the pen cap.

    (however I may be wrong somewhere as the effect is very small especially at the observation 2)

    PS:the distance between my eye and the further table edge is about 40-50cm if there is importance in that.
  5. May 25, 2010 #4
    Try focusing on the pen cap when you move it closer to your eyes. You will notice that the objects in the background move about (don't focus on them!)

    I think what your experiencing is a slight focus change as you move the pen cap closer to your eye, that gives a smaller but similar effect.
  6. May 25, 2010 #5
    I think that's approximately correct. I've noticed such effects too.

    phoebus16, how about doing experiments with stronger/weaker light? That is, see if the aperture size makes a difference.

    You can quantify the effect by using carefully positioned test objects and seeing that a slight misalignment of the edges of the Far and Medium objects can be pulled together as the Medium edge is pulled toward the Near object. It can totally hide the Far, so that color edge becomes completely blocked.

    What is the position of the Near object when this cut-off occurs, and how does that change with your pupil's diameter?
  7. May 25, 2010 #6
    It's a bit difficult for me to make such measurements, lacking measuring tools and possibly optics needed, as I hardly see it with naked eye.

    But I have managed to capture a video of it! So its clearly a general optics issue and not something specific to eyes.

    Here are the links to the videos, they are taken with a mobile phone with focus capability and the moving object is not a pen cap but a white paper.
    There is fluorescent lighting from all around and a desk lamp over the eraser and the paper.
    At first there is a printer at the back at a distance of 20cm from the camera and at about the middle is the eraser, the paper moves right and left at a distance of about 2cm from the camera:

    Video with focus fixed for long distance (unlimited):
    Notice how the barcode lines are "attracted" to the paper while the background hardly moves at all. (with my eyes the effect is like 5x)

    Video with focus fixed for very close distance:
    Here the effect is much smaller, almost not existent, you have to look very carefully on the barcode, and the paper is "pushing" the barcode lines instead of "attracting" them.
    The background is too blurry for me too tell but it looks like before, to not move at all.

    So it seems for some reason when the focus is unlimited, like I did with my eye, looking the couch, the near objects light seems like to be "attracted" from objects that are very close to the eye.

    btw I am an electronics engineer student, I know nothing about optics :P
  8. May 25, 2010 #7
    Now I see what you mean. It might be lack of interference at the very edge where the light is cut off, something that relates to the pin hole camera technique I am sure.

    I really can't say for sure though.

    Light bends around mass but not enough to be visible in your living room, there is an explanation for your effect, someone on here should have the details.
  9. May 25, 2010 #8
    If I understand the question correctly, what you're witnessing is a phenomenon known as parallax.

    You have two eyes, which each see things at a somewhat different angle with respect to each other as they are located at two different places in your head.

    If you close one, the view through the other jumps a little to the right or left, with respect to what the other was seeing a moment ago. Close the other, and the view jumps again, this time in the opposite direction.

    When you are looking at something with both of your eyes open, the visual cortex combines the two views into a single image. In effect your eyes are creating (not unlike a stereopticon) two distinct two-dimensional images and the visual cortex is recreating them as a single 3-D image.

    Normally, you don't notice this because you're so used to seeing it that you ignore it. It's when you start monkeying with it that the process becomes -you should pardon the expression- visible.

  10. May 25, 2010 #9
    Hm, I dont' think this has to do with having two eyes. I can do the same thing by just putting my finger very very close to my (one) eye while focusing on some other object. If I wiggle my finger slowly, I can see the object warp and contort. I have been curious about this as well and am interested in what is the explanation.
  11. May 25, 2010 #10


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    Light is diffracted somewhat by the edges of objects, especially sharp objects. The effect is more significant under monochromatic or near monochromatic light.
    Last edited: May 26, 2010
  12. May 26, 2010 #11
    I did some thought experiments, but did not draw it out with rulers and measurements yet. I think the principal cause can be illustrated with an example where object F (for Far) has an edge that is only just visible past the blocking edge of object M (Medium), but when object N enters the field of view, F's edge disappears.

    The geometry blocks the view of F from a vantage point that is on one side of the aperture (the eye's pupil in this case), but due to parallax, F is visible from the other side of the aperture. The lens gathers light from both sides of the aperture and the conflicting information causes fuzzyness ("circles of confusion"), but your perception sees the contrasting edge. If you then block one side of your visual field with N, you can no longer see F. The apparent distortion is (I think) perceptual, in a manner similar to the optical illusion that shows straight lines over circles and the lines look like they bend apart. If you simulate the view using ray tracing, and then "sharpen" the edges, you will get a similar effect. Where the available information changes, the result of the edge detection changes.
  13. May 26, 2010 #12
    I always assumed that was what I was seeing, but one of his examples is bending the wrong direction. It might contribute, to some of the effects, though.
  14. May 26, 2010 #13
    I am not questioning about this phenomenon, I only use one of my eyes and make the observations with that one eye only.
    If you take a careful look at the first video you will see the phenomenon I am talking about, it is visible from the camera, so it is not specific to eyes but it is a general optical phenomenon.

    This is exactly what I mean, I found it by moving my finger close to the eye too.
    In the video I used paper and eraser so that it will be clear of any temperature difference issues and the objects will be crisp in view.

    I am not sure if I understand that correctly but if that is what is happening then why the videos show that the barcode lines actually move? (no matter the background, just look the paper and the eraser)
    If the mouse pointer is placed on a line (in the first video I mean) then you can notice that when the paper comes "close" to that line, the line really leaves its place and moves!

    Thats interesting thanks :smile:
    That can explain the phenomenon. Any idea of why is it refracting the light?
  15. May 26, 2010 #14
    I believe this is diffraction not refraction.

    Refraction is the bending of light when it enters a medium with a different index of refraction, ie light entering water. This is why fish look like they are in one spot but are actually in another.

    Diffraction is the bending of light as is passes by an object.
    Last edited: May 26, 2010
  16. May 26, 2010 #15
    When I didn't have glasses and I sometimes had trouble what was on the board, I had to do that (finger going across my eyes) to see what was written. I don't know how it works, but it works.
    The way I see it, it's like the object is blocking quanta from coming to your eyes, the pressure on quanta passing through your eyes is stronger at the tip of the finger.

    Kind of like this:

    Water flowing downward, since the water pressure is high where the flow is, the water bleeds out of place.

    but what do I know.
  17. May 26, 2010 #16

    Yup I think it is diffraction, but not in the usual way it is demonstrated.


    The edge of the paper is like the edge of a slit. The waves propagate like Raza's picture.
  18. May 27, 2010 #17
    Draw a diagram tracing the rays of the edge going to either side of a lensed aperture and then converging again to a near focus. Do the same with one side of the aperture masked off. Look at the difference in the "circles of confusion" on the picture plane. If you interpret the densest image as being the actual edge, what happens to the image?

    The circle of confusion is off center. So, its center is now shifted over. Thus, the actual object appears shifted. (Remember the picture plane is inverted.)

  19. May 27, 2010 #18
    Try making a pinhole out of three fingers (thumb and first two fingers pinched together)—that really works well.

    That works because you are making the aperture smaller. A pinhole focuses perfectly. A lens has a "depth of field", and the larger the area you are gathering across, the worse it works.

    Using a smaller hole to see through, you increase the depth of field so the range of distance on either side of what your focus is set for (probably too near to see the board normally) is still focused well enough to be readable.

  20. May 27, 2010 #19
    Here is a drawing as described in my earlier explanation:

    finger illusion.png

    Blocking off part of the aperture makes the edge of the object appear to shift. Here, the red block is the "medium" object and you are focused at "far".


    (I don't know why it won't show the image full size in the message. I got it to work once before. Anybody know?)
  21. May 28, 2010 #20
    Oh, now I understand it all, that drawing helped a lot! Thank you very much, all of you :smile:
    I was searching for this explanation for long time :biggrin:
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