• Question: Can you explain how the hologram and cd case works again? I'd really like to try it!

    Asked by Anon on 30 Dec 2014.
    • Photo: Fiona Dickinson

      Fiona Dickinson answered on 30 Dec 2014:

      The cd case is a classic ‘trick’ that used to be used in old arcade machines (think wreck it ralph) to hide the giant tv in the bottom of the box(when tvs were massive), the trick is called pepper’s ghost (and wikipedia is helpful here with a couple of images http://en.wikipedia.org/wiki/Pepper's_ghost).

      Effectively you have a source of the image (a little screen hidden on the flat bottom of the cd case), and because of the angle of the case lid, some of the light is reflected towards you, and you see an image.

      The lid of the cd case effectively acts as a mirror, but because the mirror isn’t very good you only see the light bits and dark bits you don’t see very well. There is absolutely no difference in what is going on here as when you see yourself reflected in a window, except because of angles with the cd case the thing you are looking at is normally hidden from view.

      I hope this makes some sense… btw if you saw the Doctor Who show for the 50th anniversary about the making of the first Doctor Who episodes they also described the ‘teleprompter’ which also works by exactly the same method.

      As for the falling mist hologram of Dallas, this works by simply acting as a screen, if you shine a projector at a wall you see an image, since the mist is a little bit opaque, since there are particles of water vapour, this can act as a screen as well, the image looks a little 3d as the mist is not uniform. Again this works well because it is essentially a screen of falling mist, flat and not very wide. This is cool to look at but is essentially the same science as how rainbows appear in the sky.

    • Photo: Tim Stephens

      Tim Stephens answered on 7 Jan 2015:

      Great answer from Fiona, but just to add: the ‘hologram’ with the CD case is also how head-up displays in aeroplanes (and fancy cars) work.
      The source of the picture is below the screen, and it’s reflected off the window that the pilot looks through so it appears in front of them as if it’s floating. Maybe there’s a hack in there for you to play with?