C.5.1 The Doppler Effect
The Doppler effect
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When a source of sound, such as the whistle of a train or the siren of an ambulance, moves away from an observer :
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It appears to decrease in frequency, i.e. it sounds lower in pitch
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Although, the source of the sound remains at a constant frequency
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When the observer and the source of wave are both stationary :
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The waves appear to remain at the same frequency for both the observer and the source
C.5.1-1 Diagram of doppler effect (stationary)
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When the source starts to move towards the observer, the wavelength of the waves is shortened
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The sound, therefore, appears at a higher frequency to the observer
C.5.1-2 Diagram of doppler effect (moving source)
Redshift/Blueshift of EM Radiation
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In space the Doppler effect of light can be observed when spectra of distant stars and galaxies are observed, this is known as :
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Redshift if the object is moving away from the Earth
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Blueshift if the object is moving towards the Earth
C.5.1-3 Diagram of redshift
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Redshift : The fractional increase in wavelength due to the source and observer receding from each other
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Doppler redshift can be defined with equation :
C.5.2 The Doppler Equation
Doppler Equation
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When a source of sound waves moves relative to a stationary observer, the observed frequency can be calculated using the equation below :
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The ± depends on whether the source is moving towards or away from the observer
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If the source is moving towards the observer, the denominator is v - u
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If the source is moving away from the observer, the denominator is v + u
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the observer is moving relative to the source, the observed frequency can be calculated using the equation below :
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The ± depends on whether the observer is moving towards or away from the source
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If the observer is moving towards the source, the numerator is v + u
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If the observer is moving away from the source, the numerator is v − u