To: Ken Hunter 
From: jmax{at}toad.net
Cc: jmax{at}toad.net, Harold , ATM 
Reply-To: jmax{at}toad.net



Quoting Ken Hunter :

> \
> > >
> > Light doesn't interact with itself, so the intersecting beams don't
> > bother each other.
>
> Hmmm...
>
> Would you please explain Newton's rings, interference fringes and
> various other observable phenomena where light DOES appear
> to interact with itself... PLEASE
>
> Ken Hunter
>
>

Sure.

Interference phenomena are what you get when you put a detector in a region
of space containing two waves. The _detector_ responds to the sum of both
waves, but the two waves don't change each other.

Mathematically, this is expressed by the fact that the expression
describing the EM field in such a region is the sum of the two expressions
describing the individual waves. If the two waves interacted, the
expression for the region of intersection would be some more complicated
function of the two wave expressions (this really does happen for elastic
waves in some media).

As others have pointed out, this is all classical EM; if you drop down to
the quantum electrodynamics level, the equations become non-linear, and so
photons do scatter off each other... but the effect is extremely small for
any reasonable beam intensities. You _might_ have to worry about it if you
were building a terawatt laser fusion setup... and then again, maybe not.

-John



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