Hi. It’s Mr. Andersen. And this is AP Physics essentials video 109. It is on wave interference.
In other words what happens to waves when they hit objects or hit other waves? All the
animals pictured here use echo location. What does that mean? They generate sounds waves
that reflect off objects and when they come back they can figure out what they are looking
at. Now waves will also hit other waves. And as they do sometimes the overall wave becomes
larger and sometimes it becomes smaller. So let me give you an example of that. I have
a slinky here. And I am going to send two waves towards each other. Watch what happens
when they meet. So the wave becomes bigger. But did the waves bounce off each other or
did they move through one another? Well it is kind of hard to tell since both waves are
on the same side, we do not know if they bounce back or if they went through. So a better
way to figure this out is to send waves on either side. So I am going to send a wave
from the left on the top and the right on the bottom. You can see they interact but
now the wave on the bottom continues to the left. And so when waves hit they interact
but they move through each other. They are not reflected off of each other. And so wave
interactions can take place when a wave interacts with an object, hits an object or hits another
wave. So when that interaction is with another object we get reflection. It bounces back.
Now it depends on what we are hitting as to how we bounce back. And so if it is a fixed
object, the way for example a slinky hits something that does not move, it will actually
be inverted when it comes back. In other words the wave goes in this direction. It is on
the top. And when it comes back it is going to be on the bottom. That is only if it is
a fixed object. What happens if it is a free object? Well, the opposite happens. And so
as it comes in now let’s say that this is able to move back and forth, the wave comes
in on the top and it goes back on the top. And so that is reflection. Now when waves
hit each other we get what is called interference, two waves interacting. That interference can
either be constructive. Constructive is when two waves come toward each other and they
are both on the top. So what do we get when they hit? We get one big wave. We are adding
those waves together. So we are constructing a bigger wave. If it is destructive, if we
were on either side, then what would we get? We cancel each other out. So we are destroying
the size of that wave. And so we use the law superposition to figure out how big that wave
is. And it is really easy. If you are 1 centimeter on top, on both of those waves, and it is
constructive we just add 1 plus 1 and we get 2. And so as waves bounce back and forth what
we can do is we can build up what are called standing waves. Waves that appear to not move,
but what they really are is the addition of the waves bouncing back and forth. And so
let’s look at this using a PHET simulation. Let’s start with a fixed end. So remember,
if it goes into a fixed end, watch what happens as it comes back, it is inverted. So it went
down on the top and it came back on the bottom. Watch what happens if we now change that from
a wave interaction where we have a free end. So it has the ability to move up and down.
It comes back on the same side. And so again it depends on what we are hitting as to how
we come back. But we are coming back. We are being reflected. Now let’s move into interactions
between waves. When waves hit waves. And so that can be, remember, constructive or it
can be destructive. And so let’s start a wave moving down. So let me pause it for a
second. Do you remember what is going to happen? We are hitting a free end. And so what is
going to happen to that wave when it comes back? You should have said it is on the same
side. And so let’s do that. You are right. But you can see that I also generated a wave
that is now moving from the left. These are about to hit each other. And so what is going
to happen as they interact? Well we should see constructive interference. And let’s
watch what happens. So we get one big wave right when they come together. Now what is
going to happen to the wave on the left? Well it is hitting a fixed object so it should
be inverted and the one of the right should hit an object that is free and so it should
not be inverted. So now what is going to happen when they hit? You should have figured this
out. This is destructive interference now. And so watch what happens when they hit. They
should cancel each other out. And we can just see this reflection and interference and interference
and reflection again. Example of this, I am going to take a slinky and really send a pretty
big wave. We will do this in slow motion. You can see I kind of almost knocked my camera
over. So the wave is moving to the left. It hits a fixed object and it was inverted. It
hit my hand, which was a fixed object and then it is going back in the other direction.
So that is going to be that reflection of the wave. Now what will happen is if those
waves keep bouncing back and forth and back and forth and back and forth, they are adding
to and subtracting from each other. And so you get what are called these standing waves.
And this is a simbucket simulation. So you can see the green wave just appears to stand
there. It is not moving . But what is really happening is that we have waves moving back
and forth. So we have a blue wave that is moving to the right. It is reflecting back
and then it is moving to the left as this orange wave. And so as they move, they are
interfering with each other. And so let’s just grab one section right here. So why is
this low? It is because they each are moving down from equilibrium. And so we just add
those two values up. Why is the green right here at the middle? It is because we are above
and below. And so we have destructive interference here. But we have constructive interference
right here. In other words we are building up that wave. Now we take advantage of that.
So we can put sound waves that fit into a specific set length, for example a pipe in
a pipe organ, and you create what are called these harmonics of standing waves. And we
will talk about more of those in future videos. And so did you learn to make claims about
what happens when waves overlap? Remember they go through each other, but they can interfere.
And then finally could you look at a representation and figure out points when they are adding
and when they are subtracting from each other? I hope so. And I hope that was helpful.