Reflection of a Pulse
at a Free End
A GIF Animation
Consider a pulse moving through a medium - perhaps through a
rope, a slinky or even a telephone cord. What happens to the pulse
when it reaches the end of that medium? The answer depends on whether
the medium is fixed in space or free to move at its end. For example,
if the pulse is moving through a rope and the end of the rope is held
firmly by a person, then it could be said that the pulse is
approaching a fixed end. On the other hand, if the end of the rope is
lying on the floor such that it is free to move up or down when the
pulse reaches its end, then it could be said that the pulse is
approaching a free end.
The animation below depicts a pulse moving through a medium which
is free to move at its end.
The animation depicts a boundary behavior phenomenon. The
pulse reflects off the free end and returns with the same direction
of displacement which it had before reflection. That is, a pulse with
an upward displacement will reflect off the end and return with an
upward displacement. Sometimes it is said that the pulse is not
inverted. This behavior of non-inversion will always be observed
when the end of the medium is free to move. This behavior of
non-inversion is also observed when the medium is connected to
another less heavy or less dense medium. In the latter case, when the
pulse reaches the end of the medium, a portion of the pulse will
reflect off the end and return with the same direction of
displacement; it is not inverted. Since the medium beyond the
end is less dense and less heavy, it fails to fix the medium's end
and thus allows it to move; in this sense, the end of the medium
behaves as a free end.
In conclusion, a pulse reaching the end of a medium does not
become inverted whenever it either
- reflects off a free end,
- or is moving in a more dense medium and reflects off a less
dense medium.
For more information on physical descriptions of waves,
visit
The
Physics Classroom. Specific information is available there on the
following topics:
Other animations can be seen at the
Multimedia
Physics Studios. Other useful resources regarding the physics of
motion and waves is available through the
Glenbrook
South Physics Home Page.
This page was created by
Tom
Henderson of
Glenbrook South
High School.
Comments and suggestions can be sent by e-mail to
Tom
Henderson.
This page last updated on 8/11/98.