The Cosmic Classroom - The Doppler Effect

Uploaded by vmargoniner on 09.10.2009

Hi. Welcome back to the Cosmic Classroom.
Now, you should know everything about light
and and you know that light behaves
sometimes as a wave sometimes as a particle.
So I'll tell you about the Doppler Effect, which is
an effect associated with all sorts of waves, such
as sound waves for example. So I'll demonstrate sound
waves to start with. I'll turn on this really
annoying siren and you should see that if I don't
move that siren you hear the same annoying beep
over and over again. If I, if I put it in motion
when the siren is approaching, approaching you, or
receding from you, you'll notice a difference in
frequency a difference, a difference in pitch.
So let's see that. [sound]
This is the noise sound. It's not changing.
Right? So now I'll make it so that
it goes away and towards. If you heard that... ooh...
So, you probably heard that. And did you also noticed that
when I made it go faster and faster and faster it became
more and more annoying. The difference was increased.
I hope you were able to notice that.
If you didn't maybe you can rewind it and check
it out, because when it goes faster you can see a
bigger difference in frequency a bigger difference in pitch.
So this Effect occurs of with anything
that behaves like a wave. Sound is a wave,
light is also a wave. So if we go to our
first slide, is an example of an object
that it's a emitting a wave in all directions.
We've seen it from above. It's emitting waves in
all possible directions, but it's not only
emitting it's moving in this direction here.
It's moving to your left. Well, because it's
moving to your left, it's compressing the
waves this way and it's stretching the
waves behind it, that's leaving behind.
The wavelengths behind are being increased, the
light is being red shifted. The light is becoming
redder than it was originally. Well the light in this side
or the waves is in those, in this side are getting
smaller wavelengths higher frequencies and
in the case of light, they are becoming bluer.
This, this slide may do it for you, may not do it
for you so I have another analogy here.
It's just an analogy because this is really
not a wave, but it looks pretty much like a wave.
Right? It looks like a sinusoidal thing.
So lets imagine that there is a little guy here
screaming for help. Alright?
So he screams, "Help!" And help is going
in all directions. So the same pitch
is coming this direction the same wavelength
and the same, that it is going that direction.
Now if the person decides to run in my way, this way.
The pitch here has increased the wavelength
here has decreased. While the one in the
back has been stretched out, has increased.
So this is the Doppler Effect. And it happens, not on, it
happens as I said before to all kinds of lights.
Another example here, with what, with sound we have
in another picture. So lets imagine two cars.
Two cars driving in a highway. Alright?
They are both driving in the same directions, and
they are both police cars as you can see here.
So they are both imitating a sound.
They are searching for a thief or something.
So if there is a microphone here it will observe that
the frequency of sound imitated by the yellow
car will become higher. The wavelength of
those sound waves will become smaller.
And the green car that's moving away from this
microphone, the wave lengths will become
stretched, the frequency will become smaller.
It's good to use sound to visualize because
that's something you experience every day.
But it happens with light too and we know so
much about astronomy by studying the
Doppler Effect of objects that are far away from us.
So for example, this other picture here shows the
spectrum in the optical and some lines, some
absorption lines of the spectrum of a certain object.
So let's say we observe, can, we observe atomic elements
in the laboratory those are the lines that'll see.
And let's say we point our telescope
somewhere else in those lines are all shifted to
where they should be. See this line here that
is right on the yellow? It's almost at orange now.
Right? The light that is at the dark
blue it's at the very light blue. They have, they have changed,
this change tells me about the velocity of the object just
as when I was speeding the siren faster and faster
and faster there was a greater change in the frequency.
As an object moves away from you, towards me,
faster and faster I see an increase shift here.
So by observing the Doppler shift I can say
if an object is approaching me, or the lines will be
moved to the left, to the blue, or if an object is
receding from me. All the lines then
will be shifted to the right, to redder colers.
It doesn't mean that the light will become red, it
just means that it will go to redder wavelengths.
Notice that here its on yellow it continues to be
yellow it's just a little bit closer to the redder
part of the spectrum. Conversely, if it was
moving towards us all the shifts would be in
the opposite direction. Therefore, they would be
moving towards the bluer part of the spectrum.
So Doppler Effect tells us about the velocity
of objects, stars, galaxies. All the objects that are
out there are moving with respect to us.
That's why the Doppler Effect is so important
And I hope this helped.