Reflections on a transmission line

On a long straight line, waves can travel in both directions. Consider a
mechanical transmission line suspended vertically and hung from the
ceiling. Reflections is seen by setting off a pulse from the bottom of
the line. After a time equal to twice the transit time, the pulse
reflects from the ceiling and returns to your hand. This takes a few
seconds. The pulse has been reflected at the top because the line is
anchored there at zero displacement for all time. In an electronic
transmission line, this is equivalent to holding the voltage at zero for
all time by using a short circuit across the line. If you look at the
direction of displacement in the pulse, it reverses on reflection. Thus,
a wave of displacement to the right is returned as a wave of
displacement to the left. In an electronic transmission line, a square
pulse of 1 volt amplitude is returned as a square pulse of -1 volt
amplitude.