Doppler Effect

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Doppler Effect
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The Doppler Effect When a car passes you on the
street, the frequency of its engine note appears
to change. This is due to the Doppler
effect It can occur due to any relative
motion i.e. Either due to the source or the
observer moving.
The Doppler effect is the change in observed
frequency of a wave due to relative motion
between the wave source and an observer.
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The Doppler Effect
Imagine a bug jiggling its legs and bobbing up
and down in the middle of a quiet puddle. The
crests of the wave it makes are concentric
circles, because the wave speed is the same in
all directions. If the bug bobs in the water at
a constant frequency, the wavelength will be the
same for all successive waves. The wave
frequency is the same as the bugs bobbing
frequency.
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Each wave crest has a shorter distance to travel
so they arrive more frequently.
The Doppler Effect
The bug maintains the same bobbing frequency as
before. However, an observer at B would encounter
a higher frequency if the bug is moving toward
the observer.
Each crest has to travel farther than the one
ahead of it due to the bugs motion.
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Resting sound source
Frequency fs
V340m/s
Frequency fo
source at rest
observer at rest
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Sound source moving toward observer




Observer hears increased pitch (shorter wave
length)
Frequency fo
Frequency fs
source
observer at rest
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Sound source moving away from observer




Observer hears decreased pitch (longer wave
length)
Frequency fo
Frequency fs
observer at rest
source
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Observed frequency

• Source toward observer
• Increased frequency
• Source away from observer
• Decreased frequency