(Adrian Monte) What does it mean when a jet breaks
the sound barrier?
Can an object actually move faster
than the sound it creates?
In this segment, we see how the motion of objects
that emit waves affects the properties
of those waves.
It's called the Doppler effect.
♪♪
In another segment, you learned that all waves,
like light and sound waves, have different wavelengths
and frequencies.
The frequency of a wave is what we hear as pitch.
The frequency and the wavelength of a wave
are inversely proportional.
Meaning, as the wavelength of a wave increases,
its frequency decreases and vice versa.
A wave with a short wavelength
has a higher pitch,
and a wave with a longer wavelength
has a lower pitch.
Have you ever noticed that what you see
and what you hear depend on where you're standing
in relation to the sound and light waves?
In this case, what I hear,
will depend on whether I'm standing in front of
or behind this fast-moving police car.
Listen to the siren's sound
as it approaches
compared to after it goes by.
(siren wailing)
Can you hear the difference in the pitch?
The sound changes.
The pitch you hear as the car approaches...
...is higher than what the siren produces.
And after the car passes,
the pitch you hear is lower
than what the siren produces.
Why is that?
Well, you've just experienced the Doppler effect.
The Doppler effect is an increase or decrease
in the frequency of sound, light,
or other waves as the source
and the observer move toward or away from each other.
As the siren blares, it emits sound waves.
The more compressed the waves,
the shorter the observed wavelengths
and the higher the observed pitch.
So, the frequency
that someone standing in front of the moving car will hear
will be higher than the frequency
that the siren is actually emitting.
What do you think happens to the sound waves
if you're standing behind the car?
Since the sound waves are pushed closer together
in front of the car, the sound waves behind the car
are more spread out relative to the motion of the car.
That means longer wavelengths behind the car
and a lower frequency and observed pitch.
So, the sound changes depending where you're standing
relative to the motion of the car.
Here on earth, the most common types of waves
that we associate with the Doppler effect
are water and sound waves.
But, remember, the Doppler effect applies to all types of waves
where there's relative motion between the source
and observer, including light waves.
But, here on earth,
we are not able to discern a difference
in the emitted and observed frequencies
of light for moving objects.
Why is that?
Well, in order for the Doppler effect
with light waves to be noticeable,
the motion between the source
and the observer must be a significant fraction
of the speed of the wave that is traveling.
The difference between the speed of a moving object on earth
and the speed of light
is just too great for the observed frequency
to be noticeably different
than the emitted frequency.
For example,
the speed of the ambulance itself
is about 10 million times slower
than the speed of the light waves
coming from the bulb on top.
Our eyes can't see a difference in the change in frequency
produced by the Doppler effect.
It's different in space.
Astronomers are able to observe the Doppler effect
in stars or galaxies
moving either towards or away from earth
because their velocities are great enough
compared to the speed of light.
With sound,
we can hear a difference in pitch
with the frequency changes.
With light,
frequency changes correspond to changes in the color
of light that we see.
Scientists can compare the emission spectrum
seen in a star to one that's known
and determine how far the frequency of light
has shifted depending if a star or galaxy
is moving toward or away from us.
Just like these race cars and the police car...
...galaxies moving away from us
have observed lower frequencies
than the light waves the galaxy is emitting
and, therefore, longer wavelengths.
The longest wavelengths of visible light are red.
So, when the observed wavelengths are longer
than the emitted wavelengths,
this is called a red shift.
This phenomenon was first described by Edwin Hubble.
The Hubble telescope is named in his honor.
If a galaxy is moving toward us
at a very high speed,
the frequency observed will be higher
than the frequency the galaxy is emitting,
and this is called a blue shift
as the wavelength of light shifts
to the shorter wavelengths of visible light,
which are blue.
Christian Doppler discovered this shift of light frequency
back in 1842.
His discovery rocked our world
because it made so many things possible.
We can understand whether galaxies in our universe
are coming toward the earth or moving away from us.
We can gage whether a major storm is approaching,
and we can know whether or not you are speeding.
Yes, radar guns use the Doppler effect, too.
Let's say you're driving.
The police officer can aim the radar gun...
...and fire a beam of light,
usually microwaves, at your moving car.
That beam of light could bounce back at a different frequency
than initially fired
depending on the relative motion
between you and the police car.
The frequency of the light waves returning to the radar gun
will determine your speed.
Weather forecasters use the Doppler effect
to determine the speed of oncoming storms
and their intensity.
Weather radar can measure the amount of matter,
like rain, for example, in the atmosphere.
Weather radar does this by measuring the energy
of the initial signal,
and how much of that energy is reflected back
to the radar antenna.
The more energy, the more matter
in the form of precipitation.
So, Doppler radar can determine the speed
and the intensity of a storm.
You'll see that displayed as different colors
on radar weather maps.
Now, let's apply what you've learned
about the Doppler effect to this question.
What if both you and the source are moving?
To figure out how that impacts the frequency,
you must know several things--
how fast the source is moving,
which is V sub source,
how fast the observer is moving,
which is V sub observer,
how fast the wave is moving through the medium,
which we used the V by itself to represent,
and the frequency that the source is emitting,
which is F sub source.
There's an equation to determine the observed
frequency shift between an observer and a source.
Here it is.
You might notice there are plus and minus signs together
in the equations.
The direction of motion of the observer and the source
determine which one is used in a given situation.
If the observer is moving towards the source,
you would use the plus sign in the numerator.
If the observer is moving away from the source,
you would subtract.
In the denominator, you do the opposite.
If the source is moving in the direction of the observer,
you subtract.
And if it is moving away from the observer,
you add.
And if the observer isn't moving,
the observer's velocity
is zero.
And if the source isn't moving,
it's zero, as well.
Just plug that into this formula.
Let's work a problem together
to make that pretty intimidating equation
seem
(siren wailing)
A fire truck emits a frequency of 1600 hertz
from its siren.
It approaches you at a speed of 25 meters per second
while you are moving in the opposite lane
at 20 meters per second
toward the approaching fire truck.
At what frequency
do you hear the siren?
We recognize this is a Doppler effect question
because there is a difference
between the frequency that a source is emitting
and the frequency that is observed.
So, we'll have to identify our variables
to plug into the Doppler effect equation.
The frequency from the source is 1600 hertz,
as that is what the siren is emitting.
The speed of the wave is 340 meters per second,
as that is the sound in air
at what we assume is roughly room temperature.
You, the observer,
have a speed of 20 meters per second,
and the fire truck, the source,
has a speed of 25 meters per second.
The observed frequency is our unknown variable
that we are solving for.
Now that we have our variables determined,
we must plug them in to the Doppler equation.
The observed frequency
is equal to the frequency emitted by
the source, 1600 hertz,
times 340 meters per second,
plus or minus the speed of the observer,
20 meters per second,
divided by 340 meters per second,
plus or minus the speed of the source,
which is 25 meters per second.
In order to solve for the observed frequency,
we have to determine if we should add or subtract
the speeds of the observer
and the source in the equation.
Because you, the observer
are moving in the direction
towards the fire truck,
we will add the 20 meters per second
to the 340 meters per second
in the numerator.
Next, because the fire truck is moving towards you,
we have to subtract the speed of the fire truck,
25 meters per second,
from the 340 meters per second
in the denominator.
When we evaluate the equation,
we get 360 meters per second in the numerator
and 315 meters per second in the denominator.
We divide 360 meters per second
by 315 meters per second
and get a value of 1.14.
We then multiply the 1600 hertz by 1.14
to get the observed frequency
of 1,824 hertz.
And what does that mean?
That the frequency is higher,
so the pitch is higher.
As we mentioned in the beginning of this segment,
one of the biggest breakthroughs in aviation history
was breaking the sound barrier.
In 1947, Chuck Yeager became the first person to travel
faster than the sound of his jet,
faster than the speed of sound.
So, what happens when the source of sound travels faster
than the sound it's creating,
like the noise of the jet engine?
As we showed you with the car and the siren sound waves,
the same thing happens with a supersonic aircraft.
Those sound waves get compressed closer and closer
as the jet moves faster and faster.
The sound waves, along with the air,
are compressed starting at the nose of the aircraft
angled back similar to the bow waves
of a boat speeding through water.
The compression of the air and sound waves
increases the air pressure.
If you're on the ground looking up at that aircraft going by,
you would see it,
and then hear it a second or two later.
The jet is moving faster than its sound.
As the jet passes its own sound waves,
the air is compressed so much
it creates a sonic boom.
You don't hear it until that diagonal line passes you.
That's the sonic boom.
In this segment,
we've learned that we hear changes in pitch
as we move, or the source of the sound moves,
and we learned what happens
when the source moves faster
than the sound waves it's producing.
So, the next time you hear a siren a go by,
think about the Doppler effect and how the sound changes
depending where you are as it goes by.
That's it for this segment of "Physics in Motion",
and we will see you next time.
(announcer) For more practice problems,
lab activities, and note-taking guides,
check out the "Physics in Motion" toolkit.