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Doppler Effect

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Introduction

Topic 10.1 Doppler Effect (2h)

10.1.1 Describe and explain the Doppler effect

The Doppler effect is an apparent change in frequency of a source of sound (or other waves) when there is relative motion of the source and the listener. A Doppler effect is observed in all types of waves, however the Doppler effect with regards to light is fundamentally different than for sound or other types of waves that require a medium to propagate. To explain and/or derive the Doppler effect for light we must use Special Relativity. So we’ll skip that for now.

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Middle

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Now the frequency measured by the observer is:

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Do a little algebra and we get the formula given in the IB formula booklet:

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The plus/minus has been added to compensate for the direct of the source. The sign should be negative if the source is approaching the observer and positive if the source is moving away from the observer.

Now for a stationary source and a moving observer:

You may ask why would it be different if the observer or the source moves?

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Conclusion

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Where v0is the velocity of the observer.

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The wavelength is speed of sound divided by the frequency, we can then rewrite the equation as:

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This last equation is given in the IB formula booklet. The plus or minus is added to compensate for the direction of the observer. The sign should be negative if the observer is approaching the source and positive if the observer is moving away from the source.

Examples: The fundamental frequency of a train whistle is 300 Hz, and the speed of the train is 60 km/h. On a day

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This student written piece of work is one of many that can be found in our International Baccalaureate Physics section.

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