Video Transcript
In this lesson, we will learn how
to explain the differences between longitudinal and transverse waves and the motions
that produce them.
Transverse and longitudinal waves
are two types of waves that show very different motion. To start out, imagine we’re
standing in place, and we begin to move our arms up and down. Both arms move with the same kind
of motion and at the same rate, up and down and up and down. Now, let’s imagine that as this
continues, we attach one end of a spring to the floor below one of our hands and we
hold on to the other end. When this hand descends, the spring
is compressed. And when this hand rises, the
spring is stretched. As this goes on, let’s also imagine
that a rope is tied to the wall on our other side. And say further that we are able to
grab on to the free end of the rope.
Now as our hands go up and down, we
start to notice something going on in the spring and also the rope. In each one, a wave develops. But, these waves look very
different. In the spring, the coils seem to
bunch tightly together like they are here or spread farther apart. And we notice that the spread-out
parts alternate with the bunched-up parts. As we continue to watch this
spring, the places where it used to be spread out become bunched up, and the
bunched-up places become spread out.
Not only that, but we can see, as
well as feel on our hand, a wave that seems to be moving along this spring. In the rope in our other hand,
we’re also creating a wave, but we can see that it’s different. As our left hand moves up and down,
this wave travels away from us toward the wall. But the rope itself actually moves
like our hand, upwards and downwards.
So, here’s what we’re seeing
overall. In the spring, the wave is moving
along the length of the spring, in the up and down direction. We’ll call this the wave
direction. But then, the coils in the spring
also move the same way, up and down. We’ll say this is the wave
motion. For the spring, the direction of
the wave is parallel to the wave’s motion. Whenever this is true, what we have
is a longitudinal wave.
But now let’s think about the rope
in our other hand. In this case, the wave direction is
off to the right, while the wave’s motion matches the motion of our hand; it’s up
and down. So, there is a 90-degree angle
between the wave’s motion and its direction. Whenever this is true, we have
what’s called a transverse wave.
Both of these wave types carry
energy from one point to another. With the spring, when the wave
moves upwards into our hand, we can feel that transfer of energy. And when the wave on the spring
moves downward, we transfer that energy into the ground. For the transverse wave on our
rope, that energy is sent off to the right, while the rope, moving up and down,
transfers that energy. For both these kinds of waves, the
energy transferred is kinetic energy. It has to do with motion. To see all this more clearly, let’s
put a transverse wave and a longitudinal wave side by side. Here, our transverse wave is in
orange, and our longitudinal wave is in pink. Both waves have a direction from
left to right.
Like we saw earlier though, for the
transverse wave, the wave motion is actually up and down. This makes it different from a
longitudinal wave that moves parallel to the direction of the wave travel. If we look closely at these two
waves, we notice that the bunched-together parts of our longitudinal wave line up
with the high points on our transverse wave. The same thing is true of the
spread-out parts and the low points. It’s not always the case that these
points line up like this. But when they do, we say the waves
have the same wavelength.
Now, any longitudinal wave has
bunched-together parts and spread-out parts that alternate. And a transverse wave has high
points and low points that alternate. There are even names for these
parts of these two waves. The high point on a transverse wave
is called a crest. The low points are called
troughs. For a longitudinal wave, the
bunched-together parts are called compressions, while the spread-out parts are
called rarefactions. We can remember this by realizing
that at a rarefaction, the material that makes up the wave is rare or spread
out.
So far, we’ve been thinking of our
transverse wave as moving along a rope. But other sorts of transverse waves
are possible. For example, light is a transverse
wave. When light from the Sun reaches our
eye, the wave motion is at 90 degrees to the wave’s direction. And not all longitudinal waves are
formed by springs. Another sort of longitudinal wave
is sound. When we speak to one another, we
create compressions and rarefactions that alternate and allow sound energy to be
transmitted from one person to another. Knowing this about transverse and
longitudinal waves, let’s now get a bit of practice working with them.
The diagram shows an oscillating
string held between two poles. Is the wave on the string
longitudinal or transverse?
Let’s remember what these two types
of waves are. A longitudinal wave is one where
the wave moves in the same direction as the material the wave moves through. So, for example, sound waves, which
are longitudinal, push particles of air in the same direction as the wave moves
overall. Transverse waves are different. These waves travel at 90 degrees,
or perpendicularly, to the direction of the material the wave moves through. Earthquakes can create transverse
waves where Earth is moved up and down, but the wave itself travels
horizontally.
We want to answer two questions
about this diagram: what is the direction of the wave moving along the string? And in what direction does the
string itself move? We’re seeing this string at a
particular moment in time. If we wait a bit, then the string
might look like this. And then waiting a little longer,
the string will return to its original position. This tells us that any point on the
string itself is actually moving up and down. But what about the direction of the
wave on the string?
Whenever a string is tied between
two points, a wave traveling along the string moves from one tied-off point to the
other. In this string then, the wave
moving along it travels from left to right and right to left in the horizontal
direction. We found then that the string moves
vertically, while the wave itself moves horizontally. Since these two directions are at
right angles to one another, the wave moving along the string is transverse. We choose this as our answer.
Let’s look now at another
example.
The diagram shows a longitudinal
wave. Is the part of the wave in the
blue-shaded box compressed or rarefied?
Looking at this wave, we see that
some parts of it are bunched together. Other parts are spread out. Since the material is all in a line
along the direction of the wave’s motion, we see that the wave truly is
longitudinal. In fact, whenever we see
alternating bunched-up and spread-out parts in a wave, that tells us that the wave
is longitudinal. The technical name for the
bunched-together parts is “compressions.” The spread-out parts are called
“rarefactions.” Looking at the blue-shaded box, we
can see that here the wave is bunched together, which means that this is a
compression. For our answer then, we say that
the part of the wave in the blue-shaded box is compressed.
Alright, let’s go over what we’ve
learned in this lesson. In this lesson, we saw that waves
transfer energy. Also, waves can come in two types:
transverse and longitudinal. For a transverse wave, like we saw
with our rope, the wave direction is at 90 degrees to the wave motion. For our longitudinal wave, the wave
direction and wave motion are parallel. Lastly, we saw that the top of a
transverse wave is called a crest, while the bottom is called a trough. For a longitudinal wave, the
bunched-up parts are called compressions and the spread-out parts are called
rarefactions. This is a summary of transverse and
longitudinal waves.
