What is the Doppler Effect
Changes in the OBSERVED FREQUENCIES as a result of relative motion of the observer or the source
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When the source of the wave is moving away from or towards a stationary observer
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When the observer is moving away from or towards a stationary source
The observed frequency of a wave depends on the relative motion between the observer and the source. The greater
the relative speed, the greater the frequency shift.
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Redshift
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Is the result of the relative motion between an observer and an object that emits light
When the source of the light is moving away from the observer, the wavelength of the light is stretched out
(light is shifted towards the red part of visible light)
When the source is moving away, the light colour appear to be shifting towards red (caused by shift n
wavelength)
Redshift is a shift in the frequency of a photon towards a longer wavelength, or a lower energy
Blueshift happens when the light emitting source is moving towards the observer
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Doppler Effect explained
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Doppler effect can be described as the effect produced by a moving source of waves in which there is an
apparent shift in frequency when the source moves either away from or towards the observer
Austrian mathematician Christian Johann Doppler first offered an explanation for this phenomenon in 1842
Today we know that it applies to all waves
The source of the waves produces a series of wave fronts
Because the source is moving, it begins to catch up to the wave crests on one side while it moves away from
the crests on the opposite side
In the illustration below
 Observer 2 will hear the crests bunched up – high frequency
 Observer 1 will hear the crests stretched out – low frequency
 Note to the driver in the vehicle the sound will not change
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It is the effect that is produced by a moving source of waves in which it appears that the frequency is shifted
upwards as the source moves towards the observer and it appears that the frequency is shifted downwards
when the source is moving away from the observer
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light waves are perceived as colour, meaning the observer will sense bunched up waves as a bluer colour
(blueshift) and stretched out waves as a redder colour (redshift)
this is used by astronomers as they observe galaxies moving towards and away from earth
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Calculations
The relation between the frequency of the wave, the velocity of the source of the wave, the observed frequency and the
velocity of a wave is illustrated the formulae below.
These formulae are for an OBSERVER MOVING:
Observer moving towards stationary source
v+u
fo = f 

 v 
Observer moving away from stationary source
v−u 
fo = f 

 v 
fo = observed frequency
f = frequency of wave
v = velocity of the wave
u = velocity of observer
These formulae are for an SOURCE MOVING:
Source moving towards stationary observer
 v 
fo = f 

 v −u 
Source moving away from stationary observer
 v 
fo = f 

v+u 
fo = observed frequency
f = frequency of wave
v = velocity of the wave
u = velocity of source
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