Date: Wed, 14 Jan 1998 17:48:04 -0800
Message-Id: <v01520d00b0e2a58da840@[205.179.255.73]>
To: pinhole@exploratorium.edu
From: mafifi@redshift.com (Marc Afifi)
Subject: Waves question?
I don't remember, but did somebody ask how to demonstrate that a wave pulse
is not inverted on reflection when the particles at the boundary between
two media are free to move? I know it's more complicated than this
(something to do with impedance matching), but I was able to successfully
demonstrate that an upright wave pulse is reflected upright by taping two
lengths of different rubber tubing together, attaching one end to a
doorknob (the lighter tubing -- less impedance?) and sending a pulse with
my hand down the heavier tubing. At the boundary, indeed some energy was
reflected and some transmitted. The pulse that was transmitted was upright
and so was the reflected pulse. This is sort of tricky to observe since the
pulse travels rather quickly through both media and the reflected pulse
from the doorknob quickly returns to the interface between the two tubings
causing interference which quickly distorts the demonstration. I have tried
this before with a spring attached to a ring stand which was free to slide
up and down the stand with less success than the two rubber tubings seemed
to give. At first the students thought the pulse was inverted upon
reflection from the boundary, but upon careful inspection of the _first_
reflection only, it was clear that the reflected pulse was indeed upright.
I did not try attaching the thicker tubing to the doorknob and sending a
pulse down the thinner tubing to test whether or not the pulse traveling in
the opposite direction would be inverted, but I assume it would be. I'll
test it tomorrow to verify this.
At any rate, I felt this was an extremely effective way of demonstrating
that a wave pulse is reflected in phase if the particles are free to move.
Am I way out to lunch here? I'm sure, as usual, that I must be missing
something. Perhaps others have had similar successes with different
demonstrations?
Humbly yours,
-mafifi
______________________________________________________________________________
Favorite Oakland Raiders motto: Marc Afifi
Pacific Grove HS
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