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Does light bend around objects? (Not by gravity or diffraction)

  1. Apr 15, 2014 #1
    Today I was watching the full moon while I had one eye closed. There was a vertical metal rod which, by moving my head a bit, covered the moon completely, so that no direct light could hit my eye. (Yeah... I do things like that...)
    I saw a thin line of moonlight parallel to the rod right at the edge of it.
    It looks like the metal would bend the light along its surface for maybe a degree or so.
    It doesn't seem to be a property of the metal, because a painted wall has the same effect, but less bright.

    Does anyone know how this effect is called?

    Thanks for solving this mystery, it puzzles me since I was a child :-D

    Kind regards
  2. jcsd
  3. Apr 15, 2014 #2
    I believe that light bending around objects is called diffraction.
  4. Apr 15, 2014 #3


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    Indeed, this is diffraction. Not sure why you put "not by diffraction" in your title? Why do you think this is not diffraction?
  5. Apr 15, 2014 #4
    I didn't take diffraction as an explanation because
    A) the effect seems stronger on metals,
    B ) the bright glow seems to be too defined, no ripples in intensity are visible (as seen in the slot experiment) and
    C) I'm not able to find a photograph of the effect by Googling "visible diffraction". All I can find there is light split up in different wavelengths. And that's not what I see in my little experiment.
  6. Apr 16, 2014 #5


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    A) Metals are really reflective, so you are probably seeing both diffracted, as well as reflected light.
    B) To see an interference pattern, you need to be looking at coherent light. The light from the Sun is in all wavelengths and different phases and polarizations, so you wouldn't see very much in terms of dimming and brightening.
    C) Diffraction just means the light is bending around an object (see Huygen's wavelet model of light). But famously, the Young's two slit diffraction experiment (where the diffraction here means the light bends around the two slits) "proved" light is a wave, and so that's what you're going to see when you look up diffraction.
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