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Intensity of light along the iridium flare

  1. Jun 17, 2015 #1
    I have some question about iridium flares. My brains can' t process the whole picture.

    Iridium panell works like a mirror and reflects quite some percantege of light to the earth's surface, where it makes a bright spot with quite a big radeous. An observer sees a bright spot in the sky which is gradually becoming larger and then again slower, until you don t see it anymore. I can t understand why.

    Why don' t we see just a momentary flash? Similar to when somone is annoying us with a watch which reflects sunlight into our eyes.

    I was thinking that maybe it is because of rotation of the panell. I understand that if the angle of incidence is big, then the intensity will be smaller (changing with cosine function right ?). But then i thought that just a small change in the angle of panell would move this spot far away of the observer's position.

    I was also thinking that it may have something to do with atmosphere, scattering of light and all. But i don't believe that would be the case.

    My conclusion is that this spot on earth surface is not of the same intensity - getting smaller towards the edges. But i don' t understand why.

    I would be so glad if someone explains it a bit. I can't stand this enigma anymore :)

    Thank you so much, and sorry for my english.
  2. jcsd
  3. Jun 17, 2015 #2


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    One factor (though likely not the only factor) is that the sun is not a point-like light source. As the panel moves into and out of position, a changing fraction of its disc is not reflecting directly to you.
    It is the same effect that causes the penumbra of an eclipse.


  4. Jun 17, 2015 #3


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    Like so:

  5. Jun 17, 2015 #4
    Thank you so much!

    I understand it perfectly now. I had some ideas when i was looking at the solar eclipse dark spots on the earth surface, but i wasn't able to drive it to the end. Thank you so much. I don't want to bi nagging and annoying and i hope you will excuse. Light intensity graph is more like this right?


    Thank you again! I would be smashing my head for a while if you wouldn't help.
  6. Jun 17, 2015 #5


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    Yes. PhotoShop doesn't do curved lines easily. :wink:
  7. Jun 25, 2015 #6
    Hello Dave, i hope you will be reading this. I just wanted to aks you if this diagram is correct when explaining whole situation. Do you think there should be any corrections?
    Thank you :)

    internet 2.png
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