Doppler Effect


The animation above shows the sound wavefronts emanating from a moving source, such as a car.

QUESTIONS [Restart]
(Compare all characteristics to a non-moving source.)

For an observer in front of a moving source. . .

the wavelength is
shorter - Correct!
the same
longer


the frequency is
the same
lower
higher - Correct!


the speed of the wave is
higher
the same - Correct!
lower




For an observer behind a moving source. . .

the wavelength is
shorter
longer - Correct!
the same


the frequency is
higher
the same
lower - Correct!


the speed of the wave is
lower
higher
the same - Correct!



Describe a wave from a stationary source. What is observed in front of the source and behind the source?

Key: With a stationary source, the wave observed from any position equidistant from the source will be the same. They will have the same frequency, wavelength, and speed.


With a moving source, how are the waves in front of the source different from those behind it?

Key: The waves in front of the moving source have a higher frequency and a shorter wavelength. The waves behind a moving source will have a longer wavelength and a lower frequency. Both waves will have the same speed since the speed of a wave depends only on the medium in which the wave is traveling.

Credits

Physlet problem authored by Larry Martin and Tait Swenson, North Park University

based on video clip questions from Physics Cinema Classics