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Interesting experiment

  1. Sep 15, 2007 #1
    Find a fairly bright light source around your home, such as a lamp. The closer your eyes are to the lamp, the better you can see this. A couple feet should work well.

    Hold up each index finder so they are a couple inches apart, and place them only a couple inches away from one of your eyes. Close the other eye.

    You should now be looking in the direction of the lamp, through the gap in your fingertips, but your eye must be focused on the tips of your index fingers.

    Now, slowly bring your fingers together until they touch while continually focusing on the gap between your fingers.

    Just before your fingers touch, you should notice the gap "close off", almost like your fingers have been drawn together even though in reality they are not yet touching. If you have extremely steady hands, you might be able to "close the cap" and view darkness between your fingers even though they are still half a millimeter or so apart.

    Apparently, this demonstrates that light does indeed behave like a wave. When light rays are forced through the very small gap, the ridges and troughs of the rays interfere
    with each other and cancel each other out. You see darkness between your fingers even though they are not yet touching!
    Last edited: Sep 15, 2007
  2. jcsd
  3. Sep 15, 2007 #2
    You have observed diffraction of light, not unlike that present in the Young two-slit experiment. I tried your experiment, and it works well. Scrutinizing the edge of a razor blade shows similar effects as does that on any keen edge, slit or pinhole.

    You can calculate the approximate wavelength of light if you have a concentrated, monochromatic source, know the distance between your fingers, the distance to the eye being used, and the distance you head needs to move to duplicate of the observed wave pattern.
  4. Sep 15, 2007 #3
    If this were an interference effect, wouldn't you expect better results when the light source is further away (better collimated)? Comparing your description to a previous thread, the phenomenon may be due to the angular extension of the light source rather than wave effects.
    Last edited: Sep 15, 2007
  5. Sep 16, 2007 #4


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    IMO it's easier to do the experiment with a finger and thumb of one hand, not using both hands.

    It works fine with a light source a long distance away - for example bright sky. But don't look directly at the sun, of course!.

    It's easier to see the effect with your fingers close to your eyes, but it also works with your hand at arm's length from your eyes, if you have reasonably good eyesight and you know what to look for. That shows it has nothing to do with the fact that you can't focus your eyes properly on something too close to them.
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