If the wavelength is getting shorter in order for

the velocity to remain the same, the frequency has to be higher.

Do an example.

Let's just say, this is 24.

Say, velocity was 24 meters per second and

I said, the wavelength was 4 meters,

frequency was 6 inverse seconds.

Some of you may know the units of frequencies are hertz.

So when we talk about megahertz, that's millions of hertz.

So we won't worry about the details here, but just the idea.

4 times 6, 24 hours.

So now I'm saying as a paddle device moves along here,

the wavelengths are getting shorter.

Let's say, they go from four to three.

So, that means the frequency has to increase.

So I'm going to have a situation where it's 3 times 8,

that's why it shows the numbers here.

So, I can do it easily.

So, 3 times 8 is still 24.

So the velocity stays the same,

the wavelength is compressed a little bit, gotten shorter.

So the frequency has gone up and whether you realize it or not,

you probably have encountered this in real life or at least seen it in the movies.

Think about a train whistle.

If you have a train whistle and the train is coming towards,

you have sound waves coming towards you from the train whistle.

The velocity of the train is a moving source.

So, the train is a moving source with a certain velocity towards you.

The source of the sound waves, as we've been talking about the velocity

of the sound waves toward you does not change.

They are still coming towards you at the same rate.

They won't get there any faster to you.

But because it's a moving source towards you,

the movement of the train compresses the wavelengths of the sound out in

front of it as it travels away from the front of the train.

And that means shorter wave lengths, higher frequency.

So that's why when the train whistle coming toward you, you hear it.

It's a higher pitch.

Then as it passes by you, now you're behind the moving source just on my paddle

device, I was moving the other direction though.

Paddle device was moving this way.

Shorter wavelengths here, longer wavelengths behind it.

Same idea for the train whistle whether it's moving this way or that way,

it doesn't really matter.

We're pretending its coming this way.

So here comes a train toward you,

it's compressing the sound wavelengths in front.

Which means a higher frequency, higher pitched sound, as it moves by,

what do you hear?

The pitch drops [SOUND] like that, more or less.

And that's because behind the moving source as it moves away from you,

the wavelengths spread out a little bit.

Meaning that you have a bigger wavelength.

This number becomes bigger, the frequency, therefore,

has to be smaller to keep the velocity the same, the whole thing the same.

And therefore, you have a lower pitch sound.

And some of you again may know that this is something called the Doppler effect.

Doppler effect, so that's just a little side note.

And actually, there is a sort of relativistic Doppler effect.

We're not going to get to that in this course, but it is important.

The important point here is that wave speed depends on the medium.

When you have a moving source,

it does not change the speed of the wave through the medium.

It changes other things, it changes the wavelength, the frequency,

it does not change the speed of the waves.

And this is important and you may say, well,

why are we doing all this with wave stuff?

It's important, because we mentioned Einstein had two postulates,

two principles that he enunciated in his special theory of relativity.

One of them was the principle of relativity.

The second one was essentially number two here, our number two.

He called it the principle of light constancy.

And essentially, it was a that moving source is no change in the wave speed and

light was recognized as a wave.

So I said, hey, with a moving source, there's no change in the speed of light.