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Depth of mirror

  1. Mar 9, 2006 #1
    this is something that has confused me for quiet some time.

    the mirror is just a few millimeters thick. then how is it that we can use it to see depths of metres and kilometers. what is it that enables a mirror to show greater depth than its own
  2. jcsd
  3. Mar 9, 2006 #2
    Can you try to explain it further?
  4. Mar 9, 2006 #3


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    All it does is change the direction of light. It doesn't have any bearings on human perception of depth.

    - Warren
  5. Mar 11, 2006 #4
    The perception of depth has little to do which reflection, rather, that you have 2 eyes spatially seperated and a brain that combines the 2.
    However, there are powerful illusions if you "mess" with nature.
    For example, using an optical device that switches the left-eye view for the right(and vice-versa) a bizzare view is afforded.
    Foreground becomes background, and concave indentations appear convex.
  6. Mar 16, 2006 #5
    but the reflection is what we see. it is coming from a two dimensional object. then how is it that we can see if someone is standing behind us. he should be see to be standing by my side. all the things that are to be seen should be seen in a single plane not in a 3D view. why does this not happen.

    As Pallidin said, the foreground may become background, is this possible. has this been tested or is is just a hypothesis.
  7. Mar 16, 2006 #6


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  8. Mar 16, 2006 #7


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    There is one thing being overlooked here.

    A "reflection" not only generally preserves the intensity of the light being reflected, but in the ordinary case it also preserves the phase (other than a pi shift) of that light.

    Now, why is this important?

    Note that in an ordinary picture that you see in a photograph or a book, that picture is "replaying" only the intensity of the content. When you shine light to it, that is what it is reflecting back into your two eyes. A hologram, on the other hand, replay not only the intensity, but also the phase, or more accurately, the phase difference between the various objects being replayed. So when you shine light onto a hologram, it does not only reproduce the intensity, but also the phase difference between the various objects. When we view such light source, our two eyes and our optical nerve process these into various depth and perspective.

    The same thing occurs when visible light is being reflected off an ordinary plane mirror. The phase difference between the different objects are preserved upon reflection. Thus, your eyes can't tell the different (other than the left-right switching) that you are looking at the image from a mirror rather than directly at it.

  9. Mar 16, 2006 #8
    The light from the person reflects off two different points on the mirror and enters your two eyes at slightly different angles, meaning your eyes see the person at a slightly different place (you can test this by closing one eye alternately). Your brain is smart enough to tell that this means the object is located a certain distance away from you, but not smart enough to realise that the light coming to it has not travelled a straight path, but has rather been bent by the mirror. Thus you see the reflected world with full depth, but reversed right and left.

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  10. Mar 16, 2006 #9


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    Good point. Along those lines, if you are standing 5 feet from a mirror and someone is standing 5 feet behind you, the fact that the light just bounces off the mirror means that the person doesn't just appear to be 15 feet away from you - the light actually traveled 15 feet to get to you and things like parallax are preserved.

    So it is more "real" than, say, a photo.
  11. Mar 16, 2006 #10
    It has not only been tested, you can actually buy or simply build a device that uses 4 small mirrors that switch the left-eye view for the right and vice-versa. There was some university study somewhere that suggested than one should not do this for extended periods of time, as for some volunteers, the effect maintained for them for a short while even after the device was removed.
  12. Mar 16, 2006 #11
    The device is called a Pseudoscope. Very bizarre effect. I am tempted to build one. Here's one link if you are interested: http://pseudoscope.blogspot.com/

    Also here: http://www.grand-illusions.com/pseudoscope.htm [Broken]
    Last edited by a moderator: May 2, 2017
  13. Oct 4, 2010 #12
    I'm feeling tempted to build it too!!
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