A few weeks ago, I aimed a laser at a wire perpendicular and interestingly, I saw the diffraction pattern, like the picture below:

Diffraction pattern produced by laser aimed at a wire

Why is this happening? I mean, I don't have any slits and I'm aiming the laser at a wire but why is a diffraction pattern is seen on the screen

Can you please draw how the waves are diffracted?

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    $\begingroup$ Is this picture actually from the experiment you did, or is this a picture merely to illustrate what you mean by diffraction? $\endgroup$ – Jim Jul 15 '15 at 20:02
  • $\begingroup$ Floris has done this experiment and when I did that myself, I saw the same thing $\endgroup$ – David 2000 Jul 15 '15 at 20:03
  • $\begingroup$ one noticeable point about this, is that the bright fringes don't die after the forth or fifth of them and I could see about 30 bright fringes $\endgroup$ – David 2000 Jul 15 '15 at 20:04
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    $\begingroup$ The wire is an 'anti-slit' - It is indeed diffraction, and properly set up and calibrated can be used to, e.g., measure the thickness of different people's hair as a science experiment. $\endgroup$ – Jon Custer Jul 15 '15 at 20:07
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    $\begingroup$ Diffraction is seen when there is an obstruction on light's path, be it a slit or a wire. This is basically the superposition principle (or Babinet's principle). $\endgroup$ – jinawee Jul 15 '15 at 20:33

Some of the light is blocked by the wire. But the light passing immediately off the upper and lower edges of the wire's silhouette act as two point sources, which interfere with each other when they reach the screen behind the wire.

Babinet's principle says that the diffraction pattern from the edges of an opaque body is the same as that from the edges of a hole or slit of the same size. The reasoning behind this is that if you have two complementary screens, one opaque exactly where the other is transparent, then the radiation patterns of light passing through each screen must sum to the radiation pattern of the light when it is unobstructed by either screen. In order for this to be true, the patterns of each screen must be of the same amplitude but of opposite phase.

Here's a description of how to use diffraction around a wire to determine the thickness of the wire: http://www.optics.rochester.edu/workgroups/berger/EDay/EDay2008_Diffraction.pdf. Here is another account which shows how the light at the edges of the wire acts as two point sources: http://physicsed.buffalostate.edu/pubs/StudentIndepStudy/EURP09/Young/Young.html.

  • $\begingroup$ Now additional please give a explanation why an intensity distribution will be behind a single edge too. $\endgroup$ – HolgerFiedler Jul 16 '15 at 5:52
  • $\begingroup$ @HolgerFiedler: If you are talking about diffraction, Huygens' principle is usually invoked: Each point on a wave front may be considered a point-like light source, so the point at the edge of an opaque barrier emits its own waves in a circular pattern, which seem to bend around the opaque edge. There are many other ways of looking at this. Here is a thread that includes several different views: researchgate.net/post/What_causes_light_to_bend_around_edges $\endgroup$ – Ernie Jul 16 '15 at 18:36
  • $\begingroup$ So why not explain single or double slit by the same way? A slit is made from two edges, artful placed at a distance, to get the best intensity distribution as sum of the two edges. $\endgroup$ – HolgerFiedler Jul 16 '15 at 20:29
  • $\begingroup$ @Holger Fielder, diffraction or spreading out of light from a slit is equal to the sum of two single-edge diffractions only when the slit is relatively large. But when the width of the slit is comparable to the wavelength of the light, then suddenly you find much greater angles of diffraction, in comparison to which single-edge diffraction is minimal. $\endgroup$ – David Reishi May 5 '16 at 3:12

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