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The Doppler Shift:
If a source of waves is moving towards an observer, then the frequency at which the waves arrive at the observer is greater than the frequency at which the waves are produced by the source. If a source of waves is moving away from an observer, then the frequency at which the waves arrive at the observer is less than the frequency at which the waves are produced by the source. There is an upward shift of the observed frequency when the source approaches and a downward shift in frequency when the source recedes (moves away).
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When sound is produced by an object, a pattern of alternating compressions and rarefactions are created at the source location. These compressions and rarefactions move outward from the source through the surrounding medium. The diagram depicts the position of these wavefronts at an instant in time. You can think of the wavefronts as compressions. So the half-circles represent the positioning of five compressions relative to the location where they were originally produced. The direction that the source is heading is represented by The direction that the car is pointing. And the observer is represented by the stick figure.
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The half-circle wavefronts (compressions) were produced at different times. And since the car was moving, they were produced at different locations. Thus the half-circles are centered about different locations. The compressions should all be moving outward from their creation location at a constant speed (assuming the medium to be the same medium in the space surrounding the car). And if the car is moving at a constant speed, then the spacing of the compressions should be a constant spacing distance. This feature alone would rule out a couple of the choices. Now if the car is moving away from the observer (the stick figure), then the waves should arrive at a lower frequency (see the Know the Law section). This means that the wavefronts should be spaced farther apart behind the car than in front of the car so that they arrive at the observer at a lowered rate or frequency.
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