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| Doppler Effect |
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Whenever there is a relative motion between a source of waves and an observer, the frequency / wavelength of the wave as noted by the observer is different from the actual frequency / wavelength of the waves. This apparent shift in frequency / wavelength is called Doppler effect and occurs for all types of waves. |
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The apparent change of frequency and wavelength of a source of waves when there is a relative motion between the source and the observer is called Doppler Effect. |
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The Doppler Effect occurs whenever a source of sound or light moves relative to the observer. You have certainly experienced this effect with sound.
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As an example, whenever a fast moving car approaches and passes you, the sound of its siren seems to behave in a peculiar way. The siren's frequency appears to change from a high tone to a low tone as the car rushes by. |
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In other words, the frequency of the sound is raised when the source of sound approaches you and lowered when the source is moving away from you.
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The reason is simple. As the car approaches a stationary listener, the sound waves are crowded together, causing a decrease in wavelength and increase in frequency of the sound heard. After the car has passed and is moving away from the listener, the sound waves spread out. |
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As a result, the wavelength is increased and the observed frequency is decreased. A similar effect occurs with light and electromagnetic waves.
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