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Diffraction pattern?

  1. Jan 17, 2008 #1
    Diffraction pattern??

    This isn't a homework problem, but something I noticed and would like to understand. Hopefully I have posted this in the right area:

    I have a window that faces west. The other day I noticed a perfectly circular patch of light (several inches in diameter) on the wall directly opposite my window (which had curtains on it, pulled closed). At first I couldn’t figure out where the light was coming from, so I stood in front of the wall with the light pattern on my chest and walked forward to try to find the source. As I got closer to the window, the light pattern became rectangular and I realized it was coming from the vertical slit between the drapes that were covering the window. After opening the drapes, I realized the sun was low in the sky and looked like a small round ball of light. So the light on the wall was the image of the sun. As I closed the drapes again, the slit in the drapes became longer (just because of the way they fell closed). I turned around to look at the light pattern on the wall again, and there were about 10 perfect circles of light in a vertical line from floor to ceiling, all the same size, same intensity. So what was I observing? Is this diffraction through a slit? Why does the size of the slit determine how many circles I see, and why does the light pattern change from circular to rectangular the closer I get to the window (like the circles all shrink together to make a rectangle, like the slit in the drapes)?
    Last edited by a moderator: Jan 18, 2008
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  3. Jan 18, 2008 #2

    Tom Mattson

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    I'm scooting this over to the Physics area for discussion.
  4. Jan 18, 2008 #3

    Tom Mattson

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    Any chance you could duplicate this effect and post a snapshot?

    It certainly seems like it, although I can't explain the perfect circles. It sounds like there's some other effect at work here as well. Normally in single-slit diffraction you see alternating bright and dark strips in the pattern.

    Check out the image at the following link.


    In single-slit diffraction, the dark spots occur when light coming from different locations in the slit suffers a phase change due to path length difference that is exactly right for completely destructive interference. There is a wide central bright spot that is marked off by two dark spots, and those two dark spots are found by the formula [itex]a\sin(\theta)=\lambda[/itex], where [itex]a[/itex] is the slit width, [itex]\theta[/itex] is the angular displacement of the dark spots from the horizontal, and [itex]\lambda[/itex] is the wavelength of the light. This formula tells us that the central bright spot becomes wider as the slit width becomes narrower.

    It sounds to me like these circles you were seeing were inside the central bright spot of a single-slit diffraction pattern. As the slit became narrower, the bright spot became wider, thus revealing more of the circles contained within.

    Of course, that's just my best guess based on your description.

    That, I do not know.
    Last edited: Jan 18, 2008
  5. Jan 18, 2008 #4
    Interesting. Maybe there was more than one effect going on here. What I saw looked like what is shown on the link you posted, except there was no change in size or intensity between light spots, which I can't explain. Just a column of perfect circles. Hmm...

    I admit I actually cannot recreate the experiment to post a photo (I wish I could, it was pretty neat!). This is something I noticed a few months ago and have been thinking about ever since. I have since moved and do not have an unobstructed east/west window anymore. I used to live in a FLAT area, on the 7th floor of a building that faced directly west, so there were no obstructions. When the sun was a low ball in the sky, I would see this pattern.
  6. Jan 18, 2008 #5


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    Before film cameras they would make something called a http://en.wikipedia.org/wiki/Camera_obscura" [Broken], a dark room with a pin hole in one wall. This makes a large pinhole camera which forms a image of what is going on outside on the back wall.

    Perhaps this is what you are seeing. It takes a dark room and a bright day, but then you only got an image of the sun, so it sorta makes sense.
    Last edited by a moderator: May 3, 2017
  7. Jan 18, 2008 #6
    Isn't this the same phenomina that can occur during a solar eclipse, where it's fairly dark outside, and sunlight shining through the spaces between leaves on a tree cause many projections of the sun to appear on the ground below?
    Last edited by a moderator: May 3, 2017
  8. Aug 2, 2010 #7
    Re: Diffraction pattern??

    I ended up here trying to find out the very same thing. I also have a window facing west and was looking out a window that was somewhat dusty. The light from the low sun was diffracting through the palm trees and creating many perfect circles of light each one was about 30 millimeters.
    I did take a photo but as all the circles of light overlap it's not easy to see it in a still. As the branches move you are able to discern that there are many of these circles. About ten minutes earlier it was far more obvious. Next time I will shoot a video of it as it is best seen that way. But for now here is the rather fuzzy first attempt
    Last edited by a moderator: Apr 25, 2017
  9. Aug 2, 2010 #8
  10. Aug 2, 2010 #9


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    Re: Diffraction pattern??

    Yes, that's the "multiple pinhole camera" effect. No need for diffraction here. Normally we don't remark on little circular shadows on the ground underneath trees, but during a partial eclipse they become little crescents which are very striking. I've seen this once, about thirty years ago. I tried to find a picture on the net (I'm sure I've seen one in a book once) but didn't have any luck.
  11. Aug 2, 2010 #10
    Re: Diffraction pattern??

    So are you saying that these circles are in fact images of the sun itself caused by the individual palm fronds cross crossing and creating many small square holes which act as pin hole. This does not make sense to me as the holes are too big to act as a lenses. Also if they were images of the sun would they not appear with a more dynamic edge. But then again I perhaps misunderstand the term "Pin Hole Effect"

    My intuition was that the edges of the fronds were acting as a kind of diffraction magnifier. Hmmmm then again I'm have no real idea what I'm talking about.
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