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Can you really see light diffraction when looking through your fingers at a light?

  1. Jan 28, 2009 #1
    My physics master at school (1970s) said that if you hold your hand in front of your eye whilst looking at a bright light, you can see lines appear in the tiny gaps between the fingers. He said that though some people thought this was slit diffraction, they were mistaken, the effect actually being caused by something taking place in the eye. Thirty years later I find physics teachers are telling kids that this effect is slit diffraction. Who is right, and what, if any, is the real explanation of these readily observable lines?
     
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  3. Jan 28, 2009 #2
    Re: Can you really see light diffraction when looking through your fingers at a light

    Interesting question and my first impressions are that your teacher was correct.If it was slit diffraction you would expect to see a bright central maximum surrounded by subsidiary maxima and minima which is not what I observed.Also you would expect to see the pattern spread out when the slit width is reduced and I did not observe that either.I am not ruling out diffraction but at the moment I feel this is very unlikely.As to what the real explanation is ,well that requires a bit more thought.
     
  4. Jan 29, 2009 #3

    Andy Resnick

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    Re: Can you really see light diffraction when looking through your fingers at a light

    I dont think (noticible) diffractive effects will occur, although it would be easy to check- instead of looking at a bright light, put your fingers in front of a laser (or other monochromatic source) and project against a wall. Look for fringes- which will not be very stable since we can't hold our hands stable enough.

    My suspicion is that the optical effect is caused by blurring- my fingers are too close to my eye to focus on.
     
  5. Jan 29, 2009 #4

    rcgldr

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    Re: Can you really see light diffraction when looking through your fingers at a light

    Light will diffract around any relatively sharp edge, but it's not that noticeable with white light unless the edge is very sharp and the distortion layer is thin. However in monochromatic light, moderately sharp edges will cause significant diffraction, including small holes (much larger than pin holes):

    diffraction.pdf

    I seem to recall that if you use the stick from a soap bubble blower and look at black lighted source, the distortion through the open hole is significant compared to white light.

    I'm not sure what's going on with softer edges like your finger tips.
     
  6. Jan 29, 2009 #5

    mgb_phys

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    Re: Can you really see light diffraction when looking through your fingers at a light

    Simple test - if it is dffration you can take a photo of it.
    If it's an effect of you fingers trembling and your eye tracking them and the rates beating to block and unblock the source then you won't see anything in the photo
     
  7. Jan 29, 2009 #6

    ZapperZ

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    Re: Can you really see light diffraction when looking through your fingers at a light

    You cannot see a diffraction with your eye. You have a variable lens stuck in your eye. If you remove that and let the light right onto your cornea, then yes, you can.

    Zz.
     
  8. Jan 29, 2009 #7

    mgb_phys

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    Re: Can you really see light diffraction when looking through your fingers at a light

    However you can see a laser speckle pattern with your eyes
     
  9. Jan 29, 2009 #8
    Re: Can you really see light diffraction when looking through your fingers at a light

    What? Why do you think a variable lens should make it impossible to see a diffraction pattern? How do you reconcile this with the our use of spectroscopes? And what is your alternate explanation for the phenomena in question? (Unjustified absolutist positions produced the DDWFTTW fiasco..)

    At first look, the fact that the number of stable lines between my fingers depends strongly on the depth that I focus my eye would seem like evidence in favour of the diffraction interpretation.

    Oh, and cornea? Did you mean retina?
     
    Last edited: Jan 29, 2009
  10. Jan 30, 2009 #9
    Re: Can you really see light diffraction when looking through your fingers at a light

    OK, so nobody can give me a clear answer. Here's a hypothesis: it's caused by reflections. What I see myself is a number of duplicate images of the edge of my finger. Could each part of the skin, in between the cracks of the fingerprints, be flat enough to cause an individual reflection? The angle of incidence is almost parallel to the skin, so light should in theory reflect well. But I can't think why it only works when the fingers are out of focus.
     
  11. Jan 30, 2009 #10
    Re: Can you really see light diffraction when looking through your fingers at a light

    I have to agree with this guy - I thinks it has more to do with a person's sight ability than it has to do with diffraction.
     
  12. Jan 30, 2009 #11
    Re: Can you really see light diffraction when looking through your fingers at a light

    I agree too. Often when objects are up close you'll see double, since each eye sees a different angle but the large difference in angle makes it more difficult for the brain to process correctly (you really have two eyes for good depth perception... because a small difference in angle your brain can sort out).
     
  13. Jan 30, 2009 #12

    ZapperZ

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    Re: Can you really see light diffraction when looking through your fingers at a light

    Yes, I did mean the retina.

    If you put a lens after a double slit, you get back the image of the double slit at the focal point. What the lens does is do an "inverse fourier transform" of the diffraction pattern (the diffraction pattern is really a fourier transform of the slit).

    A "variable lens" is simply a means to be able to focus onto the retina. If you try to look at the light coming from a double slit with your eye directly, you'll see the double slit. You don't see interference pattern.

    Zz.
     
  14. Jan 30, 2009 #13
    Re: Can you really see light diffraction when looking through your fingers at a light

    If you had taken a moment to close one eye, hold up your fingers, and focus at different distances, then you would have see that the pattern disappears if you bring the aperture into focus. This nullifies your argument.
     
    Last edited: Jan 30, 2009
  15. Jan 30, 2009 #14

    ZapperZ

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    Re: Can you really see light diffraction when looking through your fingers at a light

    Have you tried putting a lens at the focal plane after a slit? What image do you see? The diffraction pattern?

    Zz.
     
  16. Jan 31, 2009 #15
    Re: Can you really see light diffraction when looking through your fingers at a light

    According to NASA, it is indeed a diffraction pattern:
    Zz's objection (that a diffraction pattern does not form behind a lens) requires further justification. (And, bigjohn45, it can't be fingerprints because it works between cards of paper too.)

    So far all evidence seems consistent with a Fresnel diffraction pattern. Can anyone suggest any other explanation specifically?
     
    Last edited: Jan 31, 2009
  17. Jan 31, 2009 #16
    Re: Can you really see light diffraction when looking through your fingers at a light

    Strictly speaking interference and diffraction are two different aspects of the same thing the minima of a diffraction pattern being at locations of destructive interference etc.The pattern I see bears a closer resemblance to a 2 slit interference pattern or the pattern from a grating than it does a single slit diffraction pattern, but there are puzzling features:
    1.There seems to be no intensity variation across the maxima .
    2.There seems to be no change of fringe separation when the gap between the fingers is changed.
    I think bigjohn 45 made a relevant point in post number 9 and I think reflections may play a part in this.I am not at all convinced by NASAs explanation and I feel there is more to this than meets the eye.
    bigjohn you have a lot to answer for -last night when I should have been relaxing with my martini my wife caught me acting like a total lemon looking through my fingers at the TV.
     
  18. Jan 31, 2009 #17

    ZapperZ

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    Re: Can you really see light diffraction when looking through your fingers at a light

    You had must made me spit my coffee all over the keyboard! You should be punished for this! :)

    Zz.
     
  19. Oct 5, 2010 #18
    Re: Can you really see light diffraction when looking through your fingers at a light

    But looking through fingers works.
    I mean I can very well read textual material kept just 1 cm away from my eyes when I look through hole created by joining my first three fingers. This is something which is to be taken notice of !
    You may ask where did I place my fingers when the text was just 1 cm away from my eyes?
    But I am a thin person and I manage anyhow to place my fingers in between eyes and text.
    Believe me.
    That is not just due to lenses in my eyes but it is something due to properties of light.
    And to find out what that is I had left my schools after 10 th standard at the age of just 15.
    But here in India I didn't get infrastructure to do so.
    But today I am working on it and got some results which I have published on my website.
    I believe we all can focus on some new property of light.
     
  20. May 20, 2012 #19
    Re: Can you really see light diffraction when looking through your fingers at a light

    I noticed as a kid (with my rather poor vision), that I could bring into focus distant objects simply by forming a small hole using the tips of three "fingers". In my case, I use the thumb and forefinger of my left hand and the thumb of my right hand. I may need to move the fingers relative to each other a bit to bring the object into better focus. When doing this, the hole formed by my finger tips are a few centimeters from my eye. So, I guess this phenomenon is light diffraction. Now in my old age, I find this technique useful in seeing LED displays across the room like digital clocks and the temperature setting of the air conditioner. I also use this technique to see the number of approaching buses (those that are LED or lighted work best). This draws odd glances at times, and makes me wonder if other people know about this technique / phenomenon. It seems that some of you may have notice this phenomenon. And yes, it works well for viewing text on TV screens across the room. So, I assume we are talking the same thing.

    -Rand.
     
    Last edited: May 20, 2012
  21. Aug 9, 2012 #20
    Re: Can you really see light diffraction when looking through your fingers at a light

    I've noticed this too (I need glasses), but never had any real use for it.
    However, I think that's just limiting the amount of light coming in from larger angles from the object, which makes lense focus errors smaller.

    The easiest way for me to notice this diffraction pattern is to be in a dark room, have the door *just slightly* open (easily adjusted to get a very long and thin opening) into a very bright room, and defocus as much as I can.
     
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