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Hair/wire laser diffraction interference pattern

  1. Aug 22, 2016 #1
    Hair/wire laser diffraction interference pattern - does it follow single-slit or double-slit?

    for single-slit interference, the equation nλ = d (y/D), (d is the width of the single-slit, y is the distance between central BRIGHT fringe to the nth DARK fringe, D is the distance from slit to screen)

    for double-slit interference, the equation nλ = d(y/D), (d is the width of the single-slit, y is the distance between central BRIGHT fringe to the nth BRIGHT fringe, D is the distance from slit to screen)

    How about for a laser shining on a hair (to find the thickness of hair/wire?), what does nλ = d (y/D) mean for the interference pattern?

    Experimentally, the actual interference pattern I observe in class is that it looks like a single-slit (the next bright fringe isn't very bright, quite dim) . But when I draw a diagram to show how light bends around the edges of the hair, it looks like double-slit.

    More importantly, what does y mean for the hair-laser experiment?

    thanks!
     
  2. jcsd
  3. Aug 22, 2016 #2

    TSny

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  4. Aug 23, 2016 #3

    andrevdh

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    The pattern of light forming on the other side of the hair is a result of diffraction and interference and of cause some of the undistrubed beam getting past the hair. Is this pattern formed by a single source of light?
     
  5. Aug 23, 2016 #4
    Yup it's a single source of light, and i suppose it'll be the same as single slit experiment. It's just trying to understand why, but i gotta draw out the lines to understand it. Thanks all!!
     
  6. Aug 23, 2016 #5

    TSny

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    I don't think it's easy to understand Babinet's principle by drawing rays or wavefronts. Instead, the elementary explanations that I've seen generally use an indirect argument. The link I posted gives a brief explanation. More of this type of argument can be found by web search and also here on the forum:
    https://www.physicsforums.com/threads/why-does-laser-on-a-hair-wire-creat-a-single-slit.465358/
     
  7. Aug 24, 2016 #6

    andrevdh

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    The wire presents the wavefronts coming from the laser with two edges, one on the left and one on the right.
     
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