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Cause of this diffraction?

  1. Mar 5, 2006 #1
    I love simple phenomina like this .. finding something "everyday" simple, and then trying to explain it or find a cause in scientific terms. I say it is diffraction, but I'm not sure.

    Here's the setup: In a darkened room light a small source of light, like a candle. Close one eye and with the other look at the candle flame and slowly close that eye, as if squinting. If you're like me, you will see "streamers of light" going up from the flame towards your forehead and downwards toward your mouth. There can be more than one streamer; they will fan out radially a bit from the vertical a few degrees. You can change the shape and extent of the streamers by tilting your head up and down, and changing your focus to the flame or adverting your focus from the flame a bit.

    What is the cause .. or the physics behind seeing these?

    It doesn't have to be that setup. If you train yourself you can see it other times, but this setup causes a more pronounced effect.

    It can't be caused by the narrowing slit in your eye, because it is perpendicular to the narrowing slit, and whichever way you rotate your head the streamers still go upwards to your forehead and downward towards your mouth.

    Then I though it might be diffraction around my eyelashes as my eye was closing. But I cut the eyelashes off and repeated (No! really, I just put my fingers up to my eye and held the eyelashes back and repeated; I still saw the "streamers").

    I think it is caused by something on the exterior of your eye, like maybe moisture on the cornea, as I put medical eyedrops in my eyes, and right after I do that it is more pronounced. It's nothing about the medicine, as I can get the same results by putting in non-medical drops.

    I posted something like this a couple years back .. about when it was winter and it just snowed and a cold front moved through the area, and I was out in my pickup driving around. These same streamers were coming down from the Christmas lights overhead and touching the hood of my pickup. I attributed the effect to light being diffracted around smoke particles on my windshield, or around moisture droplets in the air. I'm not sure sure now .. especially since seeing this effect, and no matter how I rotate my head it still goes up to my forehead and down toward my mouth.

    Anybody have a reason, or guess?
  2. jcsd
  3. Mar 5, 2006 #2
    Well, if the orientation of the "streamers" is always the same with respect to your head, then they're probably being caused by some filtering (or polarizing?) around your eyes. My best guess is your eyelashes are the cause. Even though you tried to cover them up, light is ligh,t and it will beat your best attempt to gaurd yourself from it with your finger. It's either that or some other kind of pattern with vertical orientation around your eyes that's causing this.
  4. Mar 5, 2006 #3
    I don't think so. If I'm careful I can barely see some with my eye pretty wide open .. my eyelashes shouldn't be effecting that. Plus, if I look up to a diffuse and wide light source, like a fairly rectangle light fixture on the bottom of a ceiling fan, the pattern changes to a wide smear of light rather than the narrow streamers. I don't see how a few eyelashes could cause this "wide" effect.

    I thought it might be something due to my nearsightedness, but it does the same whether I wear my glasses or not.
  5. Mar 7, 2006 #4


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    Staff: Mentor

    Yes it can. The streamers are orthogonal to the slit because of a spatial convolution effect, I believe. Like when you see streamers on star photos, and the caption mentions that they are due to the support structure for the mirrors in the telescope.

    I think you can find more info on this kind of effect if you google something like Fourier Optics. Pretty interesting stuff.

    BTW, the optical effect that still has me baffled is similar to yours -- some yellow streetlights put out multiple streamers that you see at night, and sometimes the streamers are stationary as you tilt your head, and sometimes they rotate at half the rotation rate of your head. Totally weird.
  6. Mar 8, 2006 #5

    Thanks for the lead. I did a google search on "fourier optics" and "spatial convolution", and it brought me to a bunch of info that's going to take me time to absorb (if I can understand it at all). It did bring me a pretty good site on optics:


    I believe though the diffraction spikes on astro photos are in the same plane as the spider support is. Look at this cool image of diffraction around the head of a pin .. the diffraction patterns are not perpendicular to the pin:


    I've seen those same streamers, they're pretty cool. It can be very pronounced when a cold front moves through and there's moisture in the air, like snow. It kinda makes for a "surreal" scene.
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