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From: cjh@petsd.UUCP (Chris Henrich)
Newsgroups: net.physics
Subject: Re: Cancelling EM waves
Message-ID: <648@petsd.UUCP>
Date: Wed, 18-Sep-85 13:30:36 EDT
Article-I.D.: petsd.648
Posted: Wed Sep 18 13:30:36 1985
Date-Received: Thu, 19-Sep-85 07:26:00 EDT
References: <543@sri-arpa.ARPA>
Reply-To: cjh@petsd.UUCP (PUT YOUR NAME HERE)
Organization: Perkin-Elmer DSG, Tinton Falls, N.J.
Lines: 34
Summary: Think about the interference pattern

[]
In article <543@sri-arpa.ARPA> DHowell.ES@Xerox.ARPA writes:
>Here's a new subject to think about, start aguments, etc.
>
>Let's say I have a device that emits an electromagnetic wave.
>Now I set up another one of these, and place ...
>                                   them half a wavelength apart so that
>they are 180 degrees out of phase, [so that] the waves will cancel.  
>Now I appear to be getting no energy out of this system, at least not
>in the form of EM waves.  I am still putting as much energy into the
>system.  All I did was move one of the devices.  What is happening to
>the energy?  

     The two waves do not exactly cancel; at some points they
do, but at other points they re-inforce each other.  So, if
the amplitude of one wave is 1, the amplitude of the combined
wave varies from 0 to 2.  What does this imply about the total
energy?  Well, the energy is a *quadratic* function of the
field strength, so it varies from 0 to 4.  Take an average,
for a rough guess, and you get 2.  Two sources, twice as much
energy.

     This can all be made very much more difficult, by trying
to decide exact patter of radiation is from each source, then
setting up and solving a messy integration problem... 

Regards,
Chris

--
Full-Name:  Christopher J. Henrich
UUCP:       ..!(cornell | ariel | ukc | houxz)!vax135!petsd!cjh
US Mail:    MS 313; Perkin-Elmer; 106 Apple St; Tinton Falls, NJ 07724
Phone:      (201) 758-7288


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