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B Why does the two concave mirror illusion looks realistic wh

  1. Dec 14, 2015 #1
    while a simple concave mirror projection does not?

    I'm really not sure where does this question go, but perhaps this is as good a place as any...
    If not, then I would appreciate a referring to the right place.

    So, I have encountered this nifty, decades old, illusion created by combining two concave mirrors : http://dev.physicslab.org/Document.aspx?doctype=3&filename=GeometricOptics_RealImages.xml

    I understand how it works and I kinda assumed that all mirrors which have a certain focal point would act similarly so their image projection would look like the object is actually there.
    I have googled a bit (youtubed actually ) and have encountered some videos on the matter:

    None comes even close to the quality of said illusion, and I wonder why is that?
    Same goes for those experiments with a lens (a long metallic rack in which you move a needle closer and farther from a focusing lens).

    In theory it should all work, but...

    Thank you very much!
  2. jcsd
  3. Dec 14, 2015 #2


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    The key to the two mirror illusion you linked to is the parallax caused by your two eyes. The double concave mirrors result in the reflection of the object coming to each eye from slightly different angles (i.e. parallax). This causes your brain to interpret the two images as one object, and at the smae time it interprets it as being closer than it should.

    The other videos have nothing to do with this particular geometry trick. They do not rely on the parallax of stereo vision.

    A more detailed explanation might be helpful, but that is the gist of it.
  4. Dec 18, 2015 #3


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    The image that you see, with two eyes or with one eye, actually forms in front of the mirror. It is a real image, in that light actually passes through a real point in space and appears to come from that point. A plane mirror image is formed behind the mirror and is virtual because there is no point through wihch the rays actually pass. Your eyes cannot tell the difference between the two images without some extra information (i.e. moving your head away from the region where the illusion works). Binocular vision or moving a single eye from side to side will both tell you that the image is out there in front of the mirror.
    This thread discusses a really impressive form of image projection (star-wars style) which, it was concluded, must be based on the similar use of a concave mirror. The guy who demos it has clearly practiced a lot.
  5. Jun 8, 2016 #4
    I know it has been some time, I just got really busy and 2 months later I simply forgot.
    Anyway, I just encountered this thread in my 'favourites' and I used to mind a little when people just disapeared on me if I helped them without saying thanks (I'm more understanding now :P) -so, thank you.
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