holographic projection in thin air? from where I come from ,they taught us in high school that it is possible to project holograms in thin air simply by illuminating the hologram with the "correct light" , and having a semi transparent medium in the path of the reflecting light "e.g. Water vapor or smoke" resulting in something like this image 
-is this true & if not is there any technology that can project such 3dImages in thin air ?
 A: The car in the picture seems to be projected with the projector in the top (2D projection). A similar 3D static car image could be accomplished with a cylindrical hologram. Any hologram can be illuminated in an appropriate way ("the correct light") to build a real image besides a virtual one.
Then, if you put a screen inside it, you are going to see a low contrast (due to light converging in other planes) picture (a view) of the 3D holographed object. The point or difference with lens imaging, like the one due to a standard projector, is that this allows to project in many planes (one at each time, or at the same time with dust/fog), as there is 3D information.
If you use dust you may project a 3D image, also low contrast, because your 3D distribution of point-like sources of light has a background due to light focusing in other points and that pass through. You'll also see your object is transparent, as you are watching light coming from every point.
In 2D lens image projections, light focusing in other planes produce blur (points become discs).
