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 fixed at its end.
The animation depicts a behavior which is referred to as inversion.
The pulse becomes inverted upon reflecting off the fixed end.
That is, a pulse with an upward displacement will reflect off the
end and return with a downward displacement. This inversion
behavior will always be observed when the end of the medium is
fixed. This inversion behavior is also observed when the medium
is connected to another more heavy or more 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 an
inverted displacement. The heavier medium acts like a fixed end
to cause the pulse to be inverted.
In conclusion, a pulse reaching the end of a medium
becomes inverted whenever it either
reflects off a fixed end,
or is moving in a less dense medium and reflects off a
more dense medium.
