Waves, Sound and Light
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 Tutorial.
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