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Blue Disk illusion

  1. Nov 1, 2015 #1
    The following picture has been posted in the Astronomy section (by Dave C426913).

    https://www.physicsforums.com/file:///C:/Users/John/AppData/Local/Temp/msohtml1/01/clip_image001.jpg [Broken]
    Source: http://www.space.com/4643-incredible-comet-bigger-sun.html

    Why does the comet fluctuate in size but the sun does not? Why does it appear to get smaller but not bigger?

    P.S. I was not able to post the image, but you can see it via the link. If you copy the picture to another document you will see the same effect i.e. the image fluctuation is not embedded.
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Nov 1, 2015 #2
    I don't know. Perhaps the color or our eyes being accustomed to the sun and nothing much bigger? I don't know.
  4. Nov 2, 2015 #3


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    I'm not sure what you're talking about. What fluctuations?
  5. Nov 2, 2015 #4

    Vanadium 50

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  6. Nov 2, 2015 #5
    The orange disk remains stable, while the blue disk repeatedly (at about 1 to 2 second intervals) "tries" to recede. The article referenced by Vanadium 50 explains it. I haven't finished studying the article yet, but it says that some people do not see the receding effect. So as I understand it, I am experiencing a receding effect because of the blue color, which my brain translates into a repeated reduction of disk size.

    I am going to do some experiments with Excel using different colors and shapes.
  7. Nov 2, 2015 #6


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    Interesting. Both images appear stable to me.
  8. Nov 2, 2015 #7


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    I used to see that effect at times, and I thought it was due to tired eyes playing a trick.
    Tthe effect I notice with the sun/comet image.
    But not on any image from the Wiki article.
  9. Nov 2, 2015 #8

    jim mcnamara

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    Hmm. I wonder if the folks who have issues seeing the effects know their Ishihara color test scores for color blindness. Color blindness comes in two flavors, and varying levels of color perceptual ability. So it is possible to have a less than perfect Ishihara number and still be able to see colors - most of the time.

    The reason I posted this is obvious - color plays a part in some of the wiki examples.


    Eight percent of white males are color blind, females less than ~1 percent. Color perception alleles live on the X chromosome, females have 2, males one. Color blindness is most common among European males.
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