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Hologram question

  1. Nov 17, 2009 #1
    Someone I know has asked about a certain property of holograms which he said he was shown while he was in school, but i can't figure out why the property should be true.

    He says that he once saw a video of a demonstration where a light (presumably a laser) was shone through a hologram, and the image could be seen on the wall behind the hologram.

    This doesn't seem to make sense to me. I thought that a lens would be needed to resolve the image onto a screen.

    Any help is appreciated!
     
  2. jcsd
  3. Nov 17, 2009 #2

    mgb_phys

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    A hologram can act like a lens.
    The pattern on the wall isn't strictly speaking an image, you can produce a hologram which will create pretty much any pattern of outgoing light rays when you shine a laser through it.
    That's how the add-on lenses for laser pointers that project a line or star or other shapes work - they are holograms.
     
  4. Nov 17, 2009 #3
    I'm still confused. Is this just like if I have say, a stars shaped object that happens to send light out from every point, then if I look at a wall next to it, I will see a blurry star?
     
  5. Nov 17, 2009 #4

    mgb_phys

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    No, the hologram splits the incoming laser beam and sends out copies of the beam at different angles.
    If you get the arragement of the angles right they make a pattern on the wall.
    It's not an image because it makes the same pattern although at a different scale at different distances.

    If you prefer you can think of the hologram as being a grid of little mirrors (or prisms) each bending the incoming light in a particular direction
     
  6. Nov 17, 2009 #5
    Okay and the pattern you see on the wall looks like the three dimensional thing that you see when you look into the hologram?
     
  7. Nov 17, 2009 #6

    mgb_phys

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    No it's just a 2d pattern of points.
    You generally can't see anything looking into this kind of hologram - it's not the same effect as projecting a transmission hologram.
     
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