Compressional Wave Terminus
Name: Brooke D.
What happens to a compressional wave when it goes
somewhere that is void of matter?
A compressional wave traveling through a material cannot propagate into a
vacuum, because the constituent particles do not have anything to "push
against". Hence the statement, "Sound cannot travel in a vacuum." One of
two things will happen -- or possibly some of each -- the
molecules/atoms/electrons at the surface of the material will be displaced
so far from their equilibrium position by the molecules/atoms behind it
that the bonds will break and molecules/atoms/electrons will be
"sputtered" into the vacuum. The photoelectric effect, where electrons are
ripped out of the surface of a metal by an applied electric field, is a
similar but not exactly the same.
If the energy of the compressional wave is not sufficient to cause
molecules/atoms/electrons to break the bonds holding the surface together,
then the restoring force will pull the particles back toward their
equilibrium position. Along this path of course will be other particles "in
the way". Then, depending upon conditions the wave may reflect in the
direction opposite of the initial wave. The wave may scatter, that is the
reflected wave may move in different directions than the initial
compressional wave, and/or the coherence of the compressional wave may be
disrupted. If that happens the coherent wave will dissipate as heat -- the
atoms/molecules/electrons moving in random directions. Which process occurs
will depend upon the details of the experiment. In the "real world", likely
all of the processes will occur to some extent.
The only waves that exist where there is no matter are electric and magnetic
waves. Compressional waves, and sound waves, cannot go beyond the end of
matter. If the matter ends abruptly, such as the outer surface of a
spaceship, the wave will reflect back in. If the matter does not have a
clear surface, like our atmosphere, some of the wave energy will reflect
back and some will throw molecules forward. A few molecules may even leave
Dr. Ken Mellendorf
Illinois Central College
It must bounce off, with inverse polarity.
Compression peaks in the forwards wave become the opposite: tension peaks
in the reflected wave.
The last matter pushed by a compression peak must swing out under it's
get pulled back by a tensile spring force from the matter behind it,
and then bump into the matter behind. it.
This re-bumping, and the tension that happened before it, start a wave
Sort like the lose end of a hanging string would do.
You should probably ask yourself how this matter holds itself together at
the boundary of the void.
A gas cannot simply end abruptly at a vacuum; something else must be
holding it back.
A liquid can end abruptly at its top (with respect to gravity) surface,
but in normal gravity the other sides would need a solid wall,
and that wall might change the reflection by being stiffer than the liquid
instead of looser,
or by locally having different density than the liquid.
The top surface of the atmosphere is not like the top surface of a liquid,
because the gas fades to vacuum so gradually that some other phenomenon
like wave-absorption or ejected mass or distorted reflection.
With a solid there is usually no problem.
It is clear that molecules in solids can pull on their neighbors, not just
These are natural opposites:
1) a wave hits an infinitely stiffer medium, and reflects with unchanged
2) a wave hits an infinitely less stiff medium (a vacuum does not push
back), and reflects with inverted polarity.
"No reflection" is the center point between the two: a wave hitting a new
medium with the same stiffness and mass density.
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Update: June 2012