I dont have a degree in science so pardon my terminology ignorance.....
Is it possible to cross a combination of different light waves/types
to create a shadow or darkness not using any solid matter to create
the shadow?
There is an old experiment called "Double Slit experiement" that
causes a shadow in the pattern of the light beams.  I am NOT looking
for this.  Essentailly I am wondering if it is possible to use
multiple types of light sources to create a shadow "mid air" without
projecting onto a surface or solid matter.

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Request for Question Clarification by hedgie-ga on 11 Apr 2006 23:08 PDT

Unfortunately  the 'this' in your question

"I am NOT looking for this"

 is ambiguous.  The double-slit geometry leads to 'destructive interference'.

Are you excluding 1) double-slit geometry (so e.g. three slits is OK)
or are you excluding any 'destructive interference' 
(which it seems you are asking about).

Are you asking 'in principle' or for practical applications?

Can the size of the 'shadow' or 'dark spot' may be very very small?

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Clarification of Question by hackmonkey-ga on 12 Apr 2006 15:53 PDT

Hmmm....well maybe my question is already answered but let me elaborate.

Is is possible to distort, shadow, or otherwise hinder human vision
over an open space.....
Example:
To create a "false" door/barrier that degrades the clarity of any
objects behind it.  Imagine a doorway..now instead of hanging a
physical wooden door that you would have to open to walk through, is
it possible to create any kind of visual distortion that would keep
someone on one side of the doorway from seeing a clear image of whats
on the other side of the doorway.  Does not need to be complete black
out, just blury or hindered vision.  So as they walk up to the portal,
they only have limited vision of whats behind this doorway but can
walk right through without running into any solid materials.  Of
course shining a spotlight directly into someones eyes would cause
them to lose sight of everything but this is not appropriate for this
application.
To break it down again... I basically want to make a screen door
without any solid matter that would keep someone from walking right
through it.

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Clarification of Question by hackmonkey-ga on 12 Apr 2006 16:01 PDT

Another clarification.  Imagine a wall of very thin tissue
paper....you can sort of makeout whats on the other side but it is not
clear.  To see whats on the other side more accurately you would have
to walk through the paper and break it.  Same effect but without solid
materials.  Gases could be used if non toxic, cheap, and do not damage
materials over long term.  I could create steam then shine a focused
beam of light through it in order to create this effect but over time,
steam would create water damage and is really not feasable in the
application I am thinking of.

That is different from the initial impression of the querry.

 For  the clarified querry, answer is:

 Yes. That is possible.

 It is possible to create an image 'in the thin air' without a solid screen.
 Keyword is 'hologram' and in the sci-fiction thy often call it 'holo'.

Here is the science-fiction example (from star wars)
http://www.geocities.com/~special_effect/sw_hologram.html


 In the real life, it is not that easy, and it needs 'a screen' but screen
 does not has to be solid.

 It was done in the lab, but currently quality of the image is such that it
 would hardly fool anyone to believe that there is a real object there.
 

Here are few details:
"To create a hologram in thin air is impossible because there is
nothing there to bounce the light off. I have seen TV shows and
commercials where they project the image by focusing several laser
beams in the center of the room. This wouldn't work either."
http://www.nvnews.net/articles/3dimagery/theory.shtml


 Sreen can be made of droplets floating in the air. It is possibly to create
 such droplets (fog) by different means.
 One of those means uses crossed strong laser beams.

 The comment I quoted above is correct: The crossed laser beams by itself
 do not create a hologram . But it was shown that very strong
 beams can ionise the air  and a tiny drop of water is formed at that spot.
 Resulting fog cloud can be use to ecatter a hologram which would
create illusion of a 3D object.

Here are few links

On making 3D images
http://www.3dimagery.com/flash_plugin.htm
 
On holograms in general
http://en.wikipedia.org/wiki/Hologram

simple tutorial
http://www.holostudios.com/holohelper/

Here is a description of a research effort. There are others.
http://web.mit.edu/invent/iow/rakkolainen.html


Rating appreciated.

Hedgie

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Clarification of Answer by hedgie-ga on 12 Apr 2006 17:52 PDT

Just wanted to correct a typo. This line:
 Resulting fog cloud can be use to ecatter a hologram which would..
should be
 Resulting fog cloud can be used to scatter a hologram which would..

The word 'scatter' is similar to 'reflect' a bit more complex:
http://omlc.ogi.edu/classroom/scat_demo/

By the way: You could also search for

SEARCH TERM : fog screen (and) fogscreen
e.g.
http://www.fogscreen.com/

Do I understand correctly that you are asking if two sources of light
can cancel each other out, as can be done with a constant sound, by
having something generate sound waves that mirror those of the first
source?

If I understand you accurately, you want to create a region of
darkness, suspended in midair, which cannot be seen through. You want
the region to be empty of matter, but *opaque* to the passage of
light. Is that correct?

   *If* that is correct, then the answer would be no. It is possible,
using wave interference -the phenomenon in the double slit experiment-
to create regions in space which are unilluminated by the light
radiating from the slits themselves (or holes, or whatever shape
aperture). But on the other hand, it's always possible to shine an
independent light source through the dark regions of the interference
pattern.

   A good way to illustrate this is the common shadow. A building,
illuminated by sunlight, casts a shadow. Yet if I stand off to one
side and look through the shadowed region, I can clearly see other
illuminated scenery beyond. Light scattering off the distant objects
passes right through the building's shadow and into my eye. The shadow
is only relative to a given light source.