medium. Air is the medium for most sound waves which reach
our ears. However, most physical substances, such as wood,
steel and water are also mediums for sound waves. Thus sound
waves may have several mediums. A vacuum is not a medium
for sound, which can be proven, by ringing a bell in a closed jar
from which the air can be pumped, and listening for sound
waves. On the other hand, Radio waves will pass through all
substances, including a vacuum, from which it is customary to
say that the ether is the medium for Radio waves; and the only
medium.
Let us now return to the “wave motion” of a rope and study
the term “velocity” so that we can apply it to sound waves and
Radio waves. It is best to study single waves first. This we can
do very well by studying the waves produced in a rope because
the motion can be confined within the medium in a single direction,
while other mediums have the motions extending in more
than one direction.
Have you ever fastened a long rope to the top of a post, then
tried to wrap the rope around the post by sending “waves” along
the rope as shown in Fig. 11? If you have, then no doubt you
noticed that the shape of the wave could be controlled by the
motion of the hand. Thus, a single wave could be sent along the
rope which kept Moving always as a “crest” or “hump” until it
reached the post. Then, upon reaching the post the “wave”
would reflect and come back on the under side of the rope Its a
“trough” Or “hollow.” Likewise, it is possible to control the
motion of the hand so that the wave will start out on the under
side of the rope as a “trough” and upon striking the post it will
reflect as a “crest,” traveling back to the hand if it has enough
energy to travel that far. Similarly, a wave can be sent along
the medium as a “crest and a trough combined,” as shown in
Fig. 11(C) by Proper movement of the hand, which will return
as a “trough and a crest” upon being reflected. (Sound waves
and Radio waves also can be reflected.)
In each case the “wave” would travel a certain distance in
the same amount of time. In other words a wave motion always
has the same speed in the same medium. In your experiments
with the rope on the floor, or tied to a post or wall, you will notice
that the waves have a definite “speed” or “velocity.” If you
change the stiffness or thickness, even by pulling on the rope,
you change its medium. When the speed is observed along a
