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Re: Waves and Energy



David Abineri wrote:

I wonder if I can get help on this question from a high school class.

If two electromagnetic waves interfere destructively, what happens to the
energy associated with the waves. I am guessing that on the large scale,
in a double slit experiment that additional energy shows up at the points
of constructive interference that compensates for the loss at the nodes.
But, what about looking as just the waves that are meeting at a node?

Any help would be appreciated. David Abineri

David:

When we think of traditional interference patterns we
are thinking in terms of monochromatic waves -
that is pure sine waves. But a pure sine wave must extend
to infinity. Therefore, the classic interference patterns
don't apply when the sources are first turned on, but
only after some sort of steady state situation has been reached.

What this would mean (if I'm correct) is that we shouldn't
take the destructive interference at the nodes too literally;
but only as an underlying mathematical construct. By thinking
of waves interfering we get the steady state pattern. But what
this means in actuality is not that two waves bring energy to
the nodes where the energy is then destroyed or redistributed
but that the electromagnetic waves along with their energy
never get to the nodes to begin with.

Think of a string with a standing wave on it. The motion
of each segment on the string is the result of the tension
of the adjacent segments. The nodes exist because at
certain segments the tensions work out in such a way as to
provide no motion. Its not that energy once existed at that
point and then was killed off.

I hope this makes sense and has some modicum of truth to it.

Ed Schweber (edschweb@ ix.netcom.com)
Physics Teacher
Solomon Schechter Day School
West Orange, NJ