Transcript Doppler Effect

Doppler Effect
Doppler Car. mpeg
• The Doppler Effect is the change in the
observed frequency caused by the motion
of the wave source.
Click to start
• Key Idea to remember, is that the wave
speed doesn’t depend on source speed.
(This is not obvious!!)
Ball: 10ms-1
How fast is the ball relative to Emma?
CAR: 20ms-1
Ball: 10ms-1
• Sound waves don’t do the same thing!
CAR: 20ms-1
SOUND:
340ms-1
SOUND: 340ms-1
Emma measures the same wave speed
whatever direction the car is moving.
Stationary Source of Waves
What do you notice about the wavelength at any point?
What do you notice about the frequency at any point?
Moving Source
What do you notice about the wavelength at any point?
What do you notice about the frequency at any point?
Stationary Source
Wavelength the same
in all directions
Moving Source
Longer
Wavelength
Shorter
Wavelength
Normal
Wavelength
When the source is approaching, the perceived wavelength is shorter
The wave speed is the same
The perceived frequency is higher
When the source is moving away, the perceived wavelength is longer
The wave speed is the same
The perceived frequency is lower
• link to simulation
Doppler Equation
The wavelength when the
source is stationary is 
The wavelength in front of
the source is  
The source frequency is f
The perceived frequency is f’
Extension
     v sT


vw
f
f
vw


vw
vw
f

vs
f

vs
f
vs
f
 vw 
 f 

 vw  vs 
The Doppler Equation is:
 vw 
f  f 

 vw  vs 
The denominator is vw
-vs if the source is moving
towards the observor
The denominator is vw
-vs if the source is moving
away from the observor
Accelerating Source
• If the source accelerates towards you, the
source speed gets gradually
• the wavelength gets gradually shorter
• So the frequency gets gradually
• If the source accelerates away fom you,
the wavelength gets gradually longer
• So the frequency gets gradually
• Sketch a graph of frequency v time for the
observer as the car passes
Constant
velocity
frequency
time
• Sketch a graph of frequency v time for the
observer as the car passes
accelerating
frequency
time
Off line Observer
Sketch a graph of frequency v time for Fleur
FLEUR
Off line Observer
What happens if Fleur is off line?
• link to simulation
FLEUR
Sketch a graph of frequency v time
for Fleur
 vw 
f  f 

 vw  vs 
In this equation, vs is the
speed the source is
approaching, not moving
Doppler Duck
Red Shift
Until about 1940, it was thought that the
universe was static.
• Hot objects emit a continuous spectrum.
When this light passes through a Hydrogen
gas, you get an absorption spectrum.
These are dark lines at certain wavelengths
Hydrogen Absorption
Spectrum as viewed in
Earth Laboratory
Hydrogen Absorption
Spectrum viewed from
a Star
What do you notice about the spectrum of light from the star?
The same spectrum reaches us, but the absorption lines
are red shifted.
This means the light has a longer wavelength.
What does this show?
This shows the galaxies are moving away from
us.
EARTH
STAR
This shows that the universe is…………
EXPANDING