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115 The Doppler Effect
Key Question: How are waves affected by the movement of the object producing them? `` The Doppler effect is something you have probably heard many
times, especially on city streets, but also anywhere there are fast moving objects.
`` Imagine a police car moving towards you at a constant speed with its siren on. The sound reaches you at a certain pitch. When the police car passes you the sound changes to a lower pitch. This is the Doppler effect in action.
`` The Doppler effect is the change in the pitch of a sound in relation to an observer who is moving relative to the source of the sound.
Wave front diagrams
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The Doppler effect is best represented by diagrams showing the waves as they are emitted or pass an observer.
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The source S is moving towards observer A. This bunches up the waves in front of the source and spreads them out behind. Observer A hears a higher pitch than observer B.
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If the source is stationary, but the observers are moving the effect is the same. Observer A (moving towards the source hears a higher pitch than observer B (who is moving away).
Uses of the Doppler effect
NASA
The Doppler effect is used to detect speed. The difference between the frequency of light waves emitted by the radar and those reflected back by the car can be used to calculate the car's speed.
Red shift is a phenomenon seen in distant galaxies. Light from galaxies moving away from us is stretched into the red part of the visible spectrum. This is used to calculate the rate of expansion of the universe.
The Doppler effect can also be used to detect blood flow in the arteries. Ultrasonic sound waves are emitted and the change in frequency can be used to detect the direct of blood flow.
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1. An observer is stationary while a siren sounding at 4000 Hz moves towards them at 20 m/s. A second observer moves at 20 m/s towards a stationary siren sounding at 4000 Hz. Explain why the observers will hear the same pitch.
2. Astronomers observe a star they know to be a yellow star similar to the Sun to be changing in color in the repeating sequence blue - yellow- red- yellow. Describe the probable motion of this star:
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