Doppler Effect using Sound Waves
The truck is producing sound waves (circles) as it moves. Each successive
wave is produced at a different position. Once a wave is produced it
spreads out in a circle from that position at the speed of sound and the
motion of the truck has no further effect on it. Since the sound waves
travel at a constant speed, the motion of the truck causes the waves
infront of the truck to be closer together (shorter wavelength, higher
frequency) and those waves behind the truck to be further apart (longer
wavelength, lower frequency).
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