Hi. It’s Mr. Andersen. And this is AP Physics essentials video 108. It is on the Doppler
Effect. You maybe heard of that before. It is why a siren, as it approaches you, you
will hear an increase in the pitch and then it will actually drop off. But it also could
occur if you were moving. And we will get into that in just a second. So let’s say
this point here is generating sound waves. They are moving away from that point. And
so if you were to listen to them, the sound waves would be hitting you at the same rate.
And so it would be a constant pitch. But watch what happens in we move that object from left
to right. As it moves from left to right you can see that the waves in front of it are
bunching up. What would that sounds like? An increase in pitch. And then behind it they
are actually spreading out. We would hear a decrease in pitch. You could see the same
thing in water waves. This swan, we know, is swimming from right to left. It is swimming
to the left. How do we know that? Well it is generating water waves. And the ones in
front of it, you can see, are getting bunched up. And the ones behind it are actually spreading
out. So that tells us it is moving to the left. It also has its beak pointed in that
direction as well. Now it is not only sound and water waves, but light is going to do
the same thing. And what one of the greatest discoveries ever was Edwin Hubble’s discovery
that when he looked out in space all light seem to be shifted to the red. We call that
a red shift. What did that mean? All the light that he saw was moving away from us. And that
indicated we were living in a universe that is constantly expanding. So Doppler effect
has these amazing repercussions. And so waves and their observed frequency will change due
to motion. And let’s say we have a source of those waves, and then we have an observer.
The source of the waves is generating waves that move out. They will be of uniform distribution
unless it is moving. So as that source moves, as it approaches the observer we are going
to see an increase in the frequency of those waves. And then as it passes the observer
we will see a decrease. But it is not only the movement of the source. It could also
be the observer. As it approaches the source it will hear an increase as it goes towards
it and then there will actually be a decrease as it moves away. And so let’s give a real
example of that. Let’s say you are standing there and a car is approaching you. The waves
as they come towards you will bunch up. And then as it moves away we are going to hear
that kind of drop off. Why does that occur? As those waves come towards you they are getting
bunched together and so the frequency is increasing. And that it is due to the change in the velocity
of that object. And so you will hear increased frequency and then it will actually drop off
as it passes you. Now we are going to experience the same thing, however, if we are moving,
the observer is moving. So analogy works good for this. Let’s say we are generating some
waves on the left side, so the waves are moving from left to right. So if you are in a boat
you would feel those waves hitting you. But let’s say now that you are moving. So the
waves are being produced but you are moving towards the waves. Since you are moving you
are going to hit them much more quickly. And so we are going to see an increase in the
frequency of the waves. Let’s say the opposite is occurring. Let’s say you are moving with
the waves. They are going to hit you at a slower rate. So you would experience a slower
frequency. Maybe a lower sound if we were talking about pitch. It also applies to light.
So as the light is moving away from you, all of those wavelengths will actually be stretched
out and that creates light that is going to be simply moving towards the red. If it is
approaching you it will actually be moving towards the blue. And so this is an example
of that red shift that Hubble saw. Since everything was shifted towards the red, everything was
moving away from us. And so did you learn to create a wave front diagram to demonstrate
why as a source approaches you the wave frequency increases? As it moves passed you it goes
away. And also, why movement of the observer can do the same thing?
I hope so. And I hope that was helpful.