Hi. It’s Mr. Andersen. And this is AP Physics essentials video 114. It is on traveling waves.
Traveling waves are waves that move from one point to another. They do not just keep bouncing
back and forth. And Jeff Rowley was able to ride this monster wave. And he was able to
actually paddle into it. But when you watch surfers like this, you will see them surfing
waves that seem big, but the weather lots of times is really nice. And so the reason
why is that these waves came from somewhere else. And so what increases the size of the
wave? Well waves are created by wind blowing over the surface of the ocean. So if you have
a fast wind blowing over the surface of the ocean then you are going to have bigger waves.
We also could have the period of time over which that wind is actually blowing. And then
we have what is called fetch. Fetch is the area over which the wind is blowing. So if
you want to get a monster wave you have to have high speed for a long period of time
over a very large area. That is going to create these traveling waves that move somewhere
else. And so waves can be traveling or standing waves. Traveling waves just like the name
implies move from one area to another. But standing waves appear to quote, unquote, stand
still. It does not look like they are moving at all. Now they actually are and what is
happening is the traveling waves are bouncing back and forth and due to interference they
are creating these stand still or standing waves. You should understand how we could
get one dimensional interference between traveling waves. And also two-dimensional traveling
wave interference. You get what is called a circular wave front. And then you can get
these interference patterns. And so let’s look at a traveling wave. This is a phet simulation.
We are creating some waves. And this magical simulation, the waves simply go out the door
and the go on forever. And so the traveling waves are created on the left and then they
just keep going and going. But what happens if we have a fixed point on the other side.
When the wave gets to the other end, it is actually reflected back. And it is inverted.
No you can see it moving to the left. And then boom, it is going to bounce back to the
right. And so these traveling waves are going back and forth and back and forth. And as
they do that they create these standing waves that appear to stand still. You will have
these little nodes and antinodes, but they are made up of traveling waves interfering
with each other, bouncing back and forth. An example of that is right here. We have
these blue waves moving to the right. We have these red waves moving back and forth. And
then we have the summation of the two. We have interference, which is creating this
big wave, areas where it is not moving much and areas where it is moving quite a bit.
And so the reason why is due to constructive and destructive interference. If we have two
waves that are stacked right on top of each other you will get these monster waves constructively
built. And then if they are out of phase with one another, what you will get is destructive
interference, where they are actually canceling each other out. Now it is not as regular as
that. As waves interact what you will get are these irregular patterns where we are
having constructive and destructive interference. These are actually beats that would be produced
by noises or pitches that are a little bit, slightly out of frequency. Now let’s go
to two-dimensions. So if we are looking at two-dimensions, in the simulation what I have
got is water being dropped which is creating these wave fronts that move out from the dropped
water. And we are measuring it in middle. But what happens if we have two drips of water
that are coming down? Now we have two wave fronts and as those wave fronts interact what
we get is interference. You can see some areas where they are canceling each other out. We
have these grey areas where there is not much waves. And then we have these darker areas,
so let me pause this right here. So what we had is if we are just measuring it along this
length, you have big waves right at the beginning. Large waves. And then it is kind of cancelled
out. We do not have as big of wave in here. And then those waves return on that side.
And example of it if we look at it in a two-dimensional kind of a simulation, if we are making waves
that are the exact same frequency you will get these huge areas where the waves simply
cancel each other out. It is called an interference pattern. And then we will have that constructive
interference where we get these monstrous waves in the middle. And so did you learn
to analyze or evaluate interactions at one-dimension, at two-dimensions as waves, traveling waves
interact with each other? I hope so. And I hope that was helpful.