Expert: Kevin Johnson - 8/25/2009

Question
QUESTION: This might be a dumb question, but can light ever be used as a surface on-which light can be projected onto, or does light absolutely always travel through other light without effecting it?

ANSWER: Hello James,

Since light has not only particle nature (it consists of photons) but it also has wave nature, it can interfere with light when certain conditions are met: The light must be correlated or coherent, meaning that it comes from the same source or it has (nearly) the same wavelength.

This does not make light like a surface (on which another beam of light can be projected), but it means that one can make light from two directions interfere with one another in such a way that the resulting light (the interference pattern) would hit a detector or the human eye in the same way as if it had originated - or had been "reflected" - at the point or area where the interference takes place.

For more detailed information on interference you might want to visit this page, and perhaps follow some of the links there:
http://en.wikipedia.org/wiki/Interference_(wave_propagation)

I hope I could help you a bit.

Peace,
 Kevin


---------- FOLLOW-UP ----------

QUESTION: So could someone use laser light fanned out as a kind of surface to project say a movie? For the sake of not requiring a screen everywhere one goes that would be a good thing as well as because I can't very well use clouds since they are set at different distances throughout.

Answer
Hello again James,

no, just providing a projection surface for a movie would not work with a "sheet" of light or laser light.

What I was talking about is something like a hologram:
http://en.wikipedia.org/wiki/Holography

i.e. you can create an interference pattern of light waves that looks like the light originated from somewhere in the air.

To project a movie you need a surface that creates diffuse reflection. This can only be done with matter, e.g. a "wall" of water vapor or "disco fog".

Peace,
 Kevin